Offshore Sarawak Research Articles

Three-Way Coupled Modeling Evaluates CO2 Sequestration in Depleted Reservoir

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 206156, “Importance of Three-Way Coupled Modeling for Carbon-Dioxide Sequestration in a Depleted Reservoir,” by Prasanna Chidambaram, SPE, Pankaj K. Tiwari, SPE, and Parimal A. Patil, SPE, Petronas, et al. The paper has not been peer reviewed. _ Three major depleted gas reservoirs in the Central Luconia field offshore Sarawak, Malaysia, are being evaluated for future carbon-dioxide (CO2) storage. A three-way coupled modeling approach that integrates dynamic, geochemistry, and geomechanics models is used to obtain the cumulative effect of all three changes. This integrated model provides a more-accurate estimate of CO2 storage capacity, caprock-integrity evaluation, CO2-plume migration path, and volume of CO2 stored through different mechanisms. Background The CO2 storage sites being evaluated are depleted gas reservoirs that have been in production for a few decades. At the end of their producing life, they have the potential to be converted into CO2 storage sites. The Central Luconia sedimentary basin is in a seismic-free zone with limited faults and consists of shale interbedded with high-sand-content sediment, making it ideal for CO2 storage. These reservoirs provide the required geological characteristics and volume needed to ensure long-term CO2 storage in a safe and economical way. The depleted gas reservoirs have an in-place volume of approximately 1.5–3 Tscf. Their thickness ranges from 100 to 150 m, with a porosity of 15–32% and permeability of 10–1600 md. These fields are believed to be supported by a regional aquifer several thousand feet thick. Large seafloor subsidence has been observed in these reservoirs. Storage-Capacity Estimation Storage capacity of a depleted hydrocarbon reservoir is affected by several factors including voidage created; aquifer influx and efflux during the production and CO2-injection phases, respectively; maximum injection pressure; rock compressibility; and geochemical effects. Depending on which of these factors are prominent in the storage reservoir, CO2-storage capacity may be estimated using a simple material-balance model or may require a more-complex approach to capture these effects. Use of the Three-Way Coupled Model Injected CO2 is anticipated to react with reservoir rock, leading to either dissolution of reservoir rock or precipitation of solids that are products of the geochemical reactions, causing a net change in porosity and permeability. With regard to geomechanical effects, uplift is anticipated to occur during CO2 injection. The degree to which subsidence is reversed depends on whether compaction of the reservoir is fully elastic or if plastic deformation has occurred. During production, reduction in porosity and permeability has occurred because of subsidence. Conventional or stand-alone reservoir simulation does not capture geochemical and geomechanical effects. Hence, it is critical to use an integrated model that captures the effects of dynamic, geochemical, and geomechanical changes caused by CO2 injection in order to evaluate suitability of the reservoir for long-term CO2 storage. A three-way coupled modeling approach that integrates dynamic, geochemistry, and geomechanics models provides a more-accurate estimate of CO2 storage capacity, along with estimation of subsidence. In three-way coupled modeling, the dynamic model is at the center, which passes input parameters to the geochemical and geomechanical models. Once it receives updated porosity and permeability values back from the geochemical and geomechanical models, the dynamic model incorporates these changes before proceeding to the next simulation timestep.

Compaction history based on porosity, pressure and temperature relation in Central Luconia province, offshore Sarawak

This study investigates the role of porosity in relation to temperature and pressure within Miocene-Pliocene sediments in Central Luconia province. The main objective is to understand the effect on porosity loss within Miocene-Pliocene sediments as part of the compaction of the sediments through time. The methodology involves seismic interpretation of 2D seismic lines and analysis of four wells over the central complex in Central Luconia province. Thick sedimentation was found within the clastic sediments of Cycle VI, indicating major clastic influx from the deltas and onshore of Sarawak during Pliocene. These clastic sediments had become a major source of weight in the offshore basin, inducing compaction within the older formations. Porosity distributions based on the wells showed that Cycle VI had the highest porosity in LH1 well with 33.77% porosity, while porosity within Cycle IV & V limestone showed an average porosity of 22% in all wells. Porosities within Cycle III and Cycle II were observed to be average of 16%. The porosity within the reservoir rock was reduced as the depth increased. This porosity reduction was relatively due to the compaction of the sediments, linked with the changes of pressure and temperature indicated by several evidences found in each well. Overpressure zone affects irregularity of porosity reduction with depth, whereas high temperature promotes diagenetic reaction that causes sudden porosity loss. Nevertheless, a sudden increase in porosity was also observed at the top of Cycle IV & V limestone due to a major drop in the sea level during the Early Late Miocene, which had encouraged the karstification process, inducing secondary porosity. This study has proved that compaction within Miocene-Pliocene sediments is not only related to the loss in porosity, but it involves changes in pressure and temperature regime of sediments and is highly dependent on the natural changes of sea level.

The crustal structure and evolution of the Bunguran Trough, offshore Sarawak, Malaysia

The Bunguran Trough is a deep Tertiary basin depocentre in the West Luconia Delta Province, Sarawak Basin, offshore Malaysia. At the centre of the trough, the top of the acoustic basement is ∼15 km deep, buried beneath a thick pile of Late Eocene to Quaternary coastal-deltaic sediments of the proto-Rajang-Lupar River delta system. Sediment thickness, bathymetry and gravity data were utilized to re-construct the crustal structure of the West Luconia Delta and the Bunguran Trough. The results indicate an average crustal thickness of 5.3 km beneath the Bunguran Trough, which is extremely thin compared to the extended continental crust (∼15–22 km) beneath the adjacent West, Central and North Luconia provinces. Extreme crustal thinning is confined to the Bunguran Trough, which appears to have steeply faulted boundaries, but the nature of the faults is obscured by the thick sedimentary pile.Structural evidences from reflection seismic data suggest widespread extensional faulting throughout Sarawak Basin although some major tectonic lineaments may have had significant strike-slip movements due to later reactivation and basin inversion. Both extensional and strike-slip faulting may have contributed to the formation and subsidence of the Bunguran Trough. West Balingian Line is a major fault zone that seems to have had a significant strike-slip displacement history and probably had an important role during basin initiation. Seismic data suggests that this fault zone extends from the Balingian and Tatau provinces into West Luconia as a major strike-slip fault with a releasing bend along which the Bunguran Trough may have developed. Crustal extension in a pull-apart basin may have been initiated by sinistral motion along the fault during Late Eocene-Oligocene. Alternatively, the widespread normal faulting indicates that the Bunguran Trough may have developed as a rift basin which experienced at least two phases of extension before a major inversion event which is marked by the Middle Miocene Unconformity (MMU). The main phase of subsidence in the Bunguran Trough had occurred during the active extensional phase before the MMU.

Low-Resistivity/Low-Contrast Reservoirs Increase Production in Offshore Brownfield

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 202331, “Unlocking the Hidden Oil: How Low-Resistivity, Low‑Contrast Reservoirs Contributed to 800 BOPD of Production in a 30‑Year‑Old Offshore Brownfield,” by Su Li Tham, SPE, Kwang Chian Chiew, and Hamed Hassani, Petronas, et al. The paper has not been peer reviewed. Production-enhancement opportunities in 30-year-old Field D in Balingian province, offshore Sarawak, are dwindling. An in-house evaluation tool, resolution enhanced modeling (REM), was developed to evaluate and characterize thin beds or laminations. These low-resistivity/low-contrast (LR/LC) sands are commonly bypassed because conventional logging tools cannot resolve their true parametric values and the apparent log responses across these zones appeared as shaly sand. By running REM across these intervals, the properties of the thin sands could be characterized properly, improving the net pay and economics of perforating and producing these reservoirs. Introduction The major reservoirs of Field D were deposited in a fluvial environment, resulting in channel sands, and are mainly under solution gas drive with weak aquifer and large gas-cap drive. The major sands generally are well-developed, including the implementation of water injection as a secondary recovery method in 2016 and perforation of remaining behind-casing opportunities across the field. As the field enters its later life, attention has been shifted to minor reservoirs in a final effort to extract remaining oil from wells before abandonment. In the case of Field D, the focus is on the LR/LC reservoirs (Fig. 1). These reservoirs mostly are shaly sands and are localized because of their depositional environment of mouth bars and crevasses. There are exceptions, however, as observed in Well A3-S and Well C6, where the reservoirs are clean sands with a thickness of 5–10 ft; these wells likely have penetrated the center of a mouth bar or crevasse. These LR/LC plays can be detected only by high-resolution logs or core photography. Consequently, LR/LC reservoirs are often overlooked because their properties and hydrocarbon saturation are underestimated, thereby placing them below the cutoff as pay zones. Thus, reservoir characterization and subsequent estimation of reserves remain a challenge in maturing LR/LC perforation proposals.

Leakage Rate Modeling in Depleted Gas Fields Safeguards Well Integrity, CO2 Storage

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 205537, “Safeguarding CO2 Storage by Restoring Well Integrity Using Leakage Rate Modeling Along the Wellbore in Depleted Gas Fields Offshore Sarawak,” by Parimal Patil, SPE, Prasanna Chidambaram, SPE, and M. Syafeeq B. Ebining Amir, Petronas, et al. The paper has not been peer reviewed. Ensuring the long-term integrity of existing plugged and abandoned (P&A) and active wells that penetrated a studied CO2 storage reservoir was key to reduction of leakage risks along the well path for long-term containment sustainability. With a goal of developing depleted gas fields as carbon dioxide (CO2) storage sites in offshore Sarawak, determining the complexity involved in restoring the integrity of these P&A wells, as well as the development wells, is critical. Leakage-rate modeling (LRM) was performed to identify and evaluate the associated risks for designing the remedial action plan. CO2 Storage Challenges in Central Luconia Storage sites in this study include three major depleted gas reservoirs in the Central Luconia gas field offshore Sarawak. Hydrocarbons from deeply buried reservoirs were produced through drilling of vertical and deviated wells. Thirty-eight wells were drilled in these three depleted gas reservoirs, of which 11 are exploration wells that had been P&A; 27 wells include active producers, water injectors, and idle wells. By estimating the performance of well-barrier elements in the legacy P&A wells and existing development wells, an assessment of well integrity analysis was performed. The casing, as well as the cement, may degrade with time depending on the downhole temperature, pressure, stress conditions, and formation fluids. Degradation of wellbore cement in the presence of carbonized acid fluids poses a risk of creating leakage pathways. Understanding the local stress conditions acting on the cement/casing/formation sheath is important to increase knowledge regarding leakage-pathway creation, together with geochemical and geomechanics processes. CO2 Storage Well Integrity Risk Assessment Once the locations of the existing P&A wells were identified, site-inspection data helped define the preparation required to assess possible gas seepage or the existence of leakage pathways along these wellbores. The measured leakage amount of gas/CO2 could then be used to backfeed the simulation model to identify the leakage system along the wellbore. The method uses desktop modeling studies together with site surveys using a remotely operated vehicle (ROV) or side scan sonar. The modeling helps simulate various leakage scenarios, while actual field measurements for any bubbling at the seabed provide the critical inputs to refine the modeling work to define the risk associated with each well. To gauge the integrity of storage sites for each well, a comprehensive understanding of critical factors and dominant criteria had to be considered as part of the data-evaluation process. Tables 1 and 2 of the complete paper define the risk-assessment elements for the existing P&A exploration and appraisal wells and development wells, respectively, used for the risk-assessment exercise. The risk-assessment process includes identification of risk, risk analysis, and risk evaluation. All wells that penetrated the CO2 storage reservoir were benchmarked as a baseline for a corrosive storage environment. Such data are useful for forecasting the possible leakage rates at the CO2 sequestration site.

Technology Focus: Mature Fields and Well Revitalization (January 2022)

The highlight of this year’s feature on mature fields and well revitalization has been the increasing use of artificial intelligence (AI) and machine learning (ML) for analysis of mature assets. Many paper proposals studied using AI/ML techniques cover a wide range of topics including optimization of production operations, faster reservoir characterization of mature fields, and improvement of ongoing waterfloods. In fact, increased use of AI/ML has been a recurring theme in many of the areas covered in the Technology Focus sections of 2021’s JPT issues. There has also been a focus on implementing novel technologies such as the use of autonomous inflow-control devices in horizontal wells to suppress ingress of unwanted fluids in thin-oil-rim reservoirs, using integrated-asset models for debottlenecking, and the use of cased-hole logging in old wells to look for missed opportunities. After serving for 5 years, my time for writing this column is coming to an end. It has been a highly rewarding exercise going through the paper proposals, experiencing the hard work put in by industry professionals to arrest inevitably declining production from mature assets by putting together ideas in diverse fields such as reservoir management, production optimization, drilling, and formation evaluation to produce the extra barrel from mature brownfields. Out of the 314 paper proposals studied in these 5 years, 137 (44%) dealt with waterflood improvement, reservoir characterization, and simulation, and approximately 78 papers (25%) dealt with well revitalization involving workover drilling, completion, and stimulation. Sustaining production from mature brownfields challenges the best from each discipline, and significant success stories have come from a genuine collaboration of ideas across disciplines from subsurface to engineering. Recommended additional reading at OnePetro: www.onepetro.org. SPE 206195 - Data-Driven Optimization of Intermittent Gas Production in Mature Fields Assisted by Deep Learning and a Population-Based Global Optimizer by Javier Fatou Gómez, TNO, et al. SPE 205546 - Role of Geomechanics in Identification of Possible Mechanisms for Nonproductive Time and Improving Drilling Operations in a Mature Field in Offshore Sarawak, Malaysia by Avirup Chatterjee, Baker Hughes, et al. SPE 206303 - Combining Capacitance Resistance Model With Geological Data for Large Reservoirs by Davud Davudov, Resermine, et al.

Technology Focus: Well Integrity (January 2022)

Almost every aspect of the upstream industry has been forced to adapt best practices and methodologies in significant ways over the last 2 years, and even the core discipline of assuring well integrity is no exception. Against a backdrop of global uncertainty and challenging economics, well integrity subject-matter experts have continued to prioritize human and environmental safety in production while pivoting their skills toward areas of growing importance such as carbon capture and sequestration. New technologies and simulation methods have allowed the authors of the three SPE papers in this year’s feature to provide a path forward that suggests new possibilities that retain the central goals of safety and productivity. Paper SPE 204385 describes the use of polymer capsules, embedded in cement and with semipermeable shells, that react with antagonistic gases downhole, such as carbon dioxide (CO2), to form a more-benign and -beneficial material. The author writes that, from this reaction with CO2, a mineral precipitate seals fractures in cement and aids in the recovery of wells. Part of the industry emphasis on decarbonization constitutes the repurposing of depleted fields as CO2 storage sites, as the authors of paper SPE 205537 describe in their discussion of such an effort offshore Sarawak. They use leakage-rate modeling to perform the critical task of estimating well integrity and re-entering existing plugged and abandoned wells. The authors of paper SPE 206052 explore various well-construction concepts and analyze the ability of cement to withstand long-term well loads. They write that, while reduction of cement stresses remains a challenge, one viable option is the use of cementless solutions, including a technology based on metal expandable packers that can seal long wellbore sections through metallic-to-rock isolation. No matter the specialized purpose or characteristics of any well, its integrity is the linchpin of production and safety. The SPE papers in this year’s feature display a versatility and willingness to tackle long-standing issues with new technologies and approaches that the upstream industry will always value. Recommended additional reading at OnePetro: www.onepetro.org. SPE 204374 - Epoxy Resin Exhibits Long-Term Durability and Chemical Stability as a Well Sealant by Fred Sabins, Riteks, et al. OTC 30299 - Big Data and Machine Learning: Effective Tools or Ensuring Well Integrity by Marcus Blane Savini, Frank’s International, et al. SPE 206252 - Development of a Digital Twin for Well Integrity Management in Underground Gas Storage Fields by Carlo Busollo, Snam, et al

Atmospheric Iron and Aluminium Deposition and Sea-Surface Dissolved Iron and Aluminium Concentrations in the South China Sea off Malaysia Borneo (Sarawak Waters)

South China Sea (SCS) is an oligotrophic sea which usually receives low nutrients supply. However, massive atmospheric dust input was occurred during the haze event in Southeast Asia for almost every year. The input of dissolved iron (DFe) and dissolved aluminium (DAl) from dust and nearby land into SCS off Sarawak Borneo region during the worst haze event in 2015 of the Southeast Asia were investigated. The estimation dust deposition during this study was 0.162 mg/m2/yr. The atmospheric fluxes of total Fe and total Al at the offshore Sarawak waters were 0.611 µmol/m2/yr and 2.03 µmol/m2/yr, respectively, where the readily available dissolved Fe and Al from the dust were 0.11 µmol/m2/yr (DFe) and 0.31 µmol/m2/yr (DAl). Fe has higher solubility (17.78%) than Al (15.21%). The lateral fluxes (e.g. from the nearby land) were 37.08 nmol/m2/yr (DFe) and 125 nmol/m2/yr (DAl), with strong Fe organic ligand class L1 (log K:22.43 – 24.33). High concentrations of DFe and DAl at the surface water of the offshore region, coincided with high concentration of macronutrients due to the prevailing south-westerly winds originated from the west Kalimantan. Low residence times, ~0.92 (DFe) and ~1.31 (DAl) years, corresponded well with DAlexcess in surface seawater due to biological utilization of DFe. Future works emphasize on natural organic Fe(III) ligands and phytoplankton study are needed for better understanding on biogeochemistry of Fe and Al at SCS off Malaysia Borneo.

Application of Spectral Decomposition Technique to Delineate the Evolution of Karst on Carbonate Platforms of Central Luconia, Offshore Sarawak, Malaysia

Karstification in carbonate platforms of the Miocene age in Central Luconia province, offshore Sarawak, Malaysia, has been discussed since the onset of exploration and initial discoveries in the region, with over 200 mapped platforms to date. An extensive drilling program over the last decade confirmed the existence of karst during the drilling process where issues such as total loss circulation and bit drops were common. Karst in Central Luconia has been proposed by several authors; however, detailed quantitative description of the observed features have not yet been conducted. This study involves systematic mapping of loss circulation depths, chalkified/rubble/vuggy zones described from cores, and vugs of >2 mm in size and moldic porosity observed on thin sections of the Jintan platform. These data supplement the interpretation of karst from multiple 3D seismic attributes. Seismic interpretation of the Jintan and M1 platforms revealed an extensive dendritic pattern which is on average 70–100 m deep and 3–5 km long, and circular geobodies of 1 km in width that exist on the upper part of the platform. Spectral decomposition, also known as time-frequency analysis, was used to enhance the interpretation of karst features on seismics within a specific wavelength. In this study, a comparison of three spectral decomposition methods applied on the 3D seismic cube of the Jintan and M1 platforms was undertaken to determine the method which allowed for better delineation of the karst features. The results show that the short-time Fourier transform (STFT) method using frequencies of 46, 54, and 60 Hz delineated most of the karst features compared to the continuous wavelet transform (CWT) Morlet and CWT Ricker wavelet methods. This paper aims to discuss the dimensions, evolution and geometry of the karst features quantitatively on three selected karst horizons named “K1”, “K2”, “K3”. Interpretation revealed that the dendritic karst features were found to be most prominent on the K2 horizon which lies below a conspicuous change of the external geomorphology of the platform. Backstepping of the platform margin by 12 km is observed in both platforms. Quantitative seismic interpretation shows that the karst observed in M1 platform is approximately 70–100 m deep, and the dendritic features are around 1–2 km in length and approximately 500 m wide; whereas, in the Jintan platform the dendritic features observed are up to 5 km in length with several 1 km wide circular/sinkhole features. More than 20 dendritic features orientated SE and NS were mapped mainly in the transitional area as well as the center of both platforms. The nature of the karst morphology in Central Luconia remains controversial; however, it is proposed to be of mixing zone karst origin.

A New Method To Estimate Resistivity Distribution Of Shaly Sand Reservoirs Using New Seismic Attributes

A subsurface resistivity model is important in hydrocarbon exploration primarily in the controlled-source electromagnetic (CSEM) method. CSEM forward modelling workflow uses resistivity model as the main input in feasibility studies and inversion process. The task of building a shaly sand resistivity model becomes more complex than clean sand due to the presence of a shale matrix. In this paper, a new approach is introduced to model a robust resistivity property of shaly sand reservoirs. A volume of seismic data and three wells located in the K-field of offshore Sarawak is provided for this study. Two new seismic attributes derived from seismic attenuation property called SQp and SQs are used as main inputs to predict the volume of shale, effective porosity, and water saturation before resistivity estimation. SQp attribute has a similar response to gamma-ray indicating the lithological variation and SQs attribute is identical to resistivity as an indicator to reservoir fluids. The petrophysical predictions are performed by solving the mathematical step-wise regression between the seismic multi-attributes and predicted petrophysical properties at the well locations. Subsequently, resistivity values are estimated using the Poupon-Leveaux (Indonesia) equation, an improvised model from Archie’s to derive the mathematical relationship of shaly sand’s resistivity to the volume and resistivity of clay matrix in shaly sand reservoirs. The resistivity modeled from the predicted petrophysical properties distributed consistently with sand distribution delineated from SQp attribute mainly in southeast, northeast, and west regions. The gas distribution of the net sand modeled by 5% and 90% of gas saturation scenarios also changed correspondingly to SQs attribute anomaly indicating the consistent fluid distribution between the modeled resistivity and SQs attribute.

Bayesian facies inversion on a partially dolomitized isolated carbonate platform: A case study from Central Luconia Province, Malaysia

We have developed a case study of geophysical reservoir characterization in which we use elastic inversion and probabilistic prediction to estimate nine carbonate lithofacies and the associated porosity distribution. The study focuses on an isolated carbonate platform of middle Miocene age, offshore Sarawak in Malaysia that has been partly dolomitized — a process that increased the porosity and permeability of the prolific gas reservoir. The nine lithofacies are defined from one reference core and include a range of lithologies and pore types, covering limestone and dolomitized limestone, each with vuggy varieties, as well as sucrosic and crystalline dolomites with intercrystalline porosity, and argillaceous limestones and shales. To predict the lithofacies and porosity from geophysical data, we adopt a probabilistic algorithm that uses Bayesian theory with an analytical solution for conditional means and covariances of posterior probabilities, assuming a Gaussian mixture model. The inversion is a two-step process, first solving for P- and S-wave velocities and density from two partial seismic stacks. Subsequently, the lithofacies and porosity are predicted from the elastic parameters in the borehole and across a 2D inline. The final result is a model that consists of the pointwise posterior distributions of the facies and porosity at each location where seismic data are available. The facies posterior distribution represents the facies proportions estimated from seismic data, whereas the porosity distribution represents the probability density function at each location. These distributions provide the most likely model and its associated uncertainty for geologic interpretations of lithofacies associated with distinct stages of carbonate platform growth.

Assessing controls on isolated carbonate platform development in Central Luconia, NW Borneo, from a regional 3D seismic facies and geomorphology investigation

Abstract The Central Luconia province in the South China Sea, offshore Sarawak, features extensive development of Middle to Late Miocene isolated carbonate platforms. This study presents a regional seismic architecture – seismic facies review of platform flank geometries and off-platform depositional styles, with the goals of understanding their patterns and exploring the controlling processes. The information on flank steepness and predominant shedding direction is contextualized with respect to extrinsic and intrinsic factors of carbonate platform growth, including tectonics, eustatic sea-level fluctuations, hydrodynamics and regional palaeogeography. Results reveal that flank geometries are consistent throughout the Middle to Late Miocene. In the north of the province eastern flanks are dominantly aggradational, steep and sediment-starved escarpments, whereas western flanks show more progradation and are accretionary in nature. Discrepancies from this pattern are observed among closely spaced platforms. The eastern-aggrading flanks are interpreted to have been highly influenced by monsoonal wind-driven currents. Further south in the province tidal currents, antecedent topography and syn-depositional tectonics were more important controls on platform architectural development. This study of regional processes and geomorphic products provides a basis for enhanced conceptual facies models and reservoir quality predictions.

Sequence biostratigraphic framework for the Oligocene to Pliocene of Malaysia: High-frequency depositional cycles driven by polar glaciation

This paper reviews quantitative biostratigraphic data from 101 petroleum exploration wells from the Malay, Sarawak and Sabah basins, and places the stratigraphy of each area into a precise chronostratigraphic framework comprising 41 transgressive-regressive depositional packages which are fully defined here. Each package is ‘fingerprinted’ by foraminiferal, nannofossil and palynological index fossils, and then tied to the global pattern of glacio-eustatic sea level and climate changes based on the global δ18O and δ13C datasets used to calibrate the geological timescale. The result is a tightly constrained chronostratigraphic framework that is applicable from deep to shallow water and is of high accuracy. It is concluded that there is a sedimentation ‘pulsebeat’ across Southeast Asia, independent of local and regional tectonics, driven by patterns of global climate change. For the Oligocene and Miocene, Antarctic glaciation is the key driver, whereas the Pliocene cycles are mainly driven by the expansion and contraction of Northern Hemisphere glaciers. The new chronostratigraphic framework allows unconformities across Malaysia to be better characterised, and the different classification schemes of seismic “Groups” in Malay and Penyu basins, “Cycles” in offshore Sarawak and “Stages” in offshore western Sabah to be correlated. Also, with a clear understanding of the effect of fluctuating sea levels on the stratigraphy of the region, it is easier to make firm judgments regarding the effect of tectonics. The study also characterises systems tracts from shelf to basin floor using biostatigraphic assemblages, which should help in ground-truthing the architecture of depositional sequences in the region based on seismic. The new framework is proposed as an alternative to the outdated TB scheme of Haq et al. (1988), widely used to interpret depositional cycles across Sarawak and Sabah.

A new approach to petroelastic modeling of carbonate rocks using an extended pore-space stiffness method, with application to a carbonate reservoir in Central Luconia, Sarawak, Malaysia

Pore geometry plays an important role in the elastic response of carbonate rocks. Diagenetic processes in carbonate sediments generate a range of pore-type distributions. Hence, the petroelastic modeling (PEM) of carbonate rocks is more complex than for clastics. Petrophysical properties connect to elastic properties through PEM or, in general terms, rock-physics modeling. Pore types cause variation in P-wave velocity — up to 40% for a given porosity. A variety of pore types with different aspect ratios such as vuggy, moldic, interparticle, intraparticle, fracture, and crack makes the porosity-velocity relationship complex, and empirical models fail to handle it properly. We propose a new, easy-to-implement approach for PEM of carbonate rocks that leads to more accurate elastic properties estimation. It offers a novel PEM method that reduces the number of defined parameters and equations. In it, the Xu-Payne rock-physics modeling equations are replaced with an extended pore-space stiffness equation. Instead of including a pore's aspect ratio as is done when using the Xu-Payne inclusion model formulation, in our proposed technique only the appropriate value of pore-space stiffness for each pore type is considered, together with the corresponding volume fraction of pore types. However, parameters are optimized by calibrating the estimated elastic properties with corresponding information from well-log measurements. This inclusion model yields acceptable predictions of elastic properties at wells that do not have measured elastic logs. The method was tested using well data from a carbonate reservoir in Central Luconia, offshore Sarawak, Malaysia. Here, one well has a complete suite of log data needed to calibrate the model. The calibrated model was then used to predict the missing shear velocity log in the other well. Next, simultaneous elastic seismic inversion was performed on 3D seismic data covering the area of the carbonate reservoir, and elastic property volumes (acoustic impedance and VP/VS ratio) were estimated. From these results, a posterior probability distribution of stiff pore types was determined, which validated the outcome of this approach using a blind test.

Unravelling an abandoned giant in Central Luconia Province, offshore Sarawak, Malaysia — Success story of Lang Lebah

In 1994, two exploration wells were drilled consecutively to explore for gas prospectivity in Lang Lebah, a Miocene carbonate buildup in the geologic province of Central Luconia located in the Sarawak Basin in Malaysia. High overpressure and operational problems prevented both wells from fully evaluating the target. Postdrill analysis concluded that Lang Lebah has limited potential due to poor reservoir quality, small gas column, and challenging drilling conditions. For these reasons, it was left dormant for 25 years. In 2016, new 3D broadband seismic acquisition and megamerge reprocessing of 3D seismic data sets followed by an integrated application of multidisciplinary workflows successfully derisked key petroleum system elements of the Lang Lebah structure, yielding a more optimistic view of its potential. A new well was justified at Lang Lebah and resulted in one of the major gas discoveries of 2019.

Investigation of fines migration for a high-pressure, high-temperature carbonate gas reservoir offshore Malaysia

A high CO2 carbonate gas field offshore Sarawak, Malaysia, is scheduled for development. Fines migration has been identified as a potential risk for a sustained and stable production for this field, triggering this investigation. The reservoir investigated (subdivided in S3, S2, S1 formation) has an average 8% clays, of which over 50% are migratory illites and 15% migratory kaolinite. This paper discusses the laboratory findings and factors contributing to fines migration for this reservoir. We conducted core flood tests to determine critical flow rates for the onset of fines migration of high CO2 hydrocarbon gas and CO2-saturated brine within the production gas zone. In addition, we measured total suspended solids during each incremental rate stage and performed scanning electron microscopy–energy-dispersive X-ray (SEM–EDX) analysis on the effluents collected. We also performed mineralogical analysis of formation rocks to determine clay types, amount, and morphology. Core floods performed with CO2-saturated brine on S3 and S2 cores showed damage at a critical flow rate of 10 ml/min with permeability recovery of 65–75%. This shows that CO2-saturated brine tends to mobilize the fines such as illite and kaolinite. Core floods performed with high CO2 hydrocarbon gas showed permeability recovery of 93–96% at rates up to 10 ml/min. SEM analysis on effluents collected showed presence of silicate and carbonate particles with size of 5–50 μm. A combined salt dropout and fines mobilization core flood showed a higher degree of formation damage due to fines mobilization with 74% permeability recovery compared to baseline permeability compared to core floods with high CO2 hydrocarbon gas. However, the formation damage due to fines mobilization for this experiment was reversible. Overall, the results indicate that a high potential for fines migration in this type of carbonate system exists for CO2-saturated brine flow with significant less potential for dry CO2 flow.

High resolution 3D seismic undershooting acquisition over platforms and seismic processing challenges in a gas producing field

One of the many challenges of seismic acquisition associated with existing fields is existence of surface platforms. The block where the seismic operation was conducted is located in offshore Sarawak and there are 3 discovered gas fields and a number of near-field exploration targets. The legacy 3D seismic survey covering the block was conducted in 1992 with limited acquisition specifications, such as short cable length and low volume airguns, in comparison to the technology of today. Hence new 3D seismic acquisition with broadband processing was commissioned which included the undershooting operation across 2 production facilities to improve the seismic imaging of the deeper exploration targets and to optimize well engineering. Overall, the new 3D data were acquired with a successful undershooting operation followed by survey merging and matching in the processing stage. The new 3D data show better seismic re?ection continuity, a higher signal to noise （S/N） ratio, clearer images, and a more accurate velocity model in the deep section, allowing for a better interpretation of the deep exploration targets.

Data-Acquisition Optimization Maximizes Value of Mature Oil-Rim Reservoirs

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 196208, “Value Creation From the Reservoir, Well, and Facilities Management (RWFM) Planning in Multistacked Mature Oil-Rim Reservoir, Offshore Sarawak, Malaysia,” by Yeek Huey Ho, SPE, Nor Baizurah Ahmad Tajuddin, SPE, Muhammed Mansor Elharith, SPE, Hui Xuan Dan, Kwang Chian Chiew, Kok Liang Tan, SPE, Raj Deo Tewari, SPE, and Rahim Masoudi, SPE, Petronas, prepared for the 2019 SPE Annual Technical Conference and Exhibition, Calgary, 29 September–2 October. The paper has not been peer previewed. The XamXung field offshore Sarawak, Malaysia, is a 47-year brownfield with thin remaining oil rims that have made field management challenging. The dynamic oil-rim movement has been a key subsurface uncertainty, particularly with the commencing of a redevelopment project. A reservoir, well, and facilities-management (RWFM) plan was implemented to optimize development decisions. This paper is a continuation of paper SPE 174638 and outlines the outcome of the RWFM plan and the results’ effect on development decisions such as infill well placement and gas/water injection-scheme optimization (Fig. 1). Key decisions affected by the RWFM findings are highlighted in the complete paper. Introduction The XamXung field was discovered in 1967, with commercial production established in 1972. The field is a simple faulted anticline bounded by two major faults in the north and south. The field consists primarily of clastic deposits characterized by thick sands interbedded with thin shale layers of late Miocene to Pliocene age. The field consists of multiple stacked gas and oil reservoirs, with key producing intervals being the oil-rim reservoirs XE/XF and XH/XI, and the deeper oil reservoir XL, as well as major nonassociated gas reservoirs XA, XC, XD, XM, and XN. The main discussion of the complete paper focuses on XE/XF and XH/XI reservoir redevelopment. XE/XF is a saturated oil-rim reservoir with initial oil-rim thicknesses of 95 and 85 ft, respectively. The XE/XF oil rim is overlain by a gas cap with m size (ratio of initial free-gas reservoir volume to initial reservoir oil volume) of 1.1 and 0.4, respectively. XE and XF reservoirs are separated by a fieldwide sealed shale layer of 10–20-ft thickness. Initial reservoir pressure and well log data acquired during the early production his-tory suggested that XE/XF reservoirs originally shared a common gas/oil con-tact (GOC) but different free-water levels. XE/XF reservoirs have an average porosity of 27–32% and average permeability of 500–1500 md. They started production in 1974 and reached peak production in 1977. The reservoir pressure has declined by only 200 psi after 45 years of production, indicating that the reservoir is under a strong aquifer drive. Gas injection into XE reservoir commenced in 1995 following an increasing water-cut trend in most of the producers, with the objective to counteract the aquifer influx and to allow uniform gas-cap gas expansion throughout the reservoir. However, the reservoir historically has produced more free gas compared with the amount of gas reinjected, resulting in the reservoir gas-cap shrinkage.

E&P Notes (September 2019)

E&P Notes (September 2019)

What matters for flow and recovery in carbonate gas reservoirs: Insights from the mature Central Luconia Province, offshore Sarawak, Malaysia

Miocene carbonate reservoirs in Central Luconia, offshore Sarawak, Malaysia, have been delivering gas for over 30 yr. In this paper, learnings from that period of production are used to understand the key drivers affecting flow during production and recovery optimization in existing fields as well as development decisions for new discoveries. The large data set, generated over more than 40 yr, was analyzed in a consistent manner through a holistic database, constrained by a stratigraphic framework, to allow reservoir units to be compared like-for-like (“integrated knowledge base” [IKB] concept). Carbonate reservoir heterogeneities impacting flow are grouped into “horizontal–heterogeneities”—argillaceous flooding layers and exposure-related karst—and “vertical–heterogeneities”—large-scale architectural elements, found especially along platform margins. Both types of heterogeneities control water ingress during production and influence the recovery mechanism. Argillaceous flooding layers can act as baffles, holding back water rise during production, or can form pressure compartments. Long-lived, fault-bounded reef margins, carbonate shoals, islands, and karsts can be vertical conduits for aquifer inflow. Platform shape and architecture impact column height and hence recovery efficiency. Additional drivers impacting recovery were found to be gas-column height, aquifer size and permeability, pressure connection to neighboring fields, and field development concepts. All drivers identified impact decisions throughout the field life, e.g., well count and design, intervention capabilities, evaluation and mitigation of early-water breakthrough, reservoir management, selecting enhanced recovery methods, and abandonment pressure. The IKB allowed to derive “big rules” on what matters for flow, which were used to decide on development strategies for greenfields in Central Luconia. The presented outcomes can be extrapolated to comparable carbonate systems, whereas the IKB approach can be adapted and applied to other mature basins and reservoir types where equally vast and historic data sets are awaiting to be used in the current era of digitalization.