Tight Carbonate Reservoir Research Articles

Experimental investigation of the impacts of supercritical carbon dioxide on acid-etching fracture morphology and conductivity in tight carbonate reservoir

Carbon dioxide (CO2) as energized fluid has been used for acid-fracturing in tight carbonate reservoir. CO2 is often pumped into the wellbore in liquid phase with acid systems, and transited into supercritical state under the high-temperature formation. While the effect of supercritical CO2 (SC-CO2) on acid-etched fracture morphology and conductivity have rarely been investigated. The acid-etching and conductivity experiments were conducted on the outcrops of the Ma5 sub-member of the Ordo Basin in China. The mixed phase reaction simulation device was developed firstly. These experiments involved the mixing of SC-CO2 with various fluids, including formation water and viscoelastic surfactant (VES) acid, as well as different SC-CO2 concentrations with VES acid. The results show that 100 % SC-CO2 is easy to activate natural fractures without dissolution. After 20%SC-CO2 mixed with formation water, the dissolution ability of the mixed solution and conductivity is only 11.8 % and 1 ∼ 2 % of VES acid respectively. Roughness of the morphology created by the mixture of SC-CO2 with VES acid, exhibits a reduction compared to VES acid-etching, accompanied by the presence of wormholes. Additionally, there is a decrease in conductivity and increase in its retention rate. With the increase of SC-CO2 concentration, the roughness of acid-etching fracture decrease and the injection pressure curve becomes more stable. While the reaction rate is also retarded nearly linearly as the SC-CO2 concentration increases. Especially, when the SC-CO2 concentration is up to 40 %, the acid-rock reaction rate reaches only 55 % of that of VES acid. The conductivity of mixed-acid-etched fracture exhibits a segmented characteristic. During the low-medium closure pressure stage, the acid-etched fracture by VES acid generates high conductivity, specifically when it is below 34.5 MPa. While the higher the SC-CO2 concentration in the mixed acid is, the greater the conductivity and retention rate are at pressures above 34.5 MPa during high closure pressure stage. The conductivity of SC-CO2 acid-fracturing fracture depends on the non-uniform etching of the acid solution. The increase in SC-CO2 concentration of the mixed acid not only enhances conductivity and retention rate under high closure pressure, but also reduces the acid volume required for acid-fracturing.

Control of strike-slip faults on Sinian carbonate reservoirs in Anyue gas field, Sichuan Basin, SW China

The largest Precambrian gas field (Anyue gas field) in China has been discovered in the central Sichuan Basin. However, the deep ancient Ediacaran (Sinian) dolomite presents a substantial challenge due to their tightness and heterogeneity, rather than assumed large-area stratified reservoirs controlled by mound-shoal microfacies. This complicates the characterization of “sweet spot” reservoirs crucial for efficient gas exploitation. By analyzing compiled geological, geophysical and production data, this study investigates the impact of strike-slip fault on the development and distribution of high-quality “sweet spot” (fractured-vuggy) reservoirs in the Ediacaran dolomite of the Anyue gas field. The dolomite matrix reservoir exhibits low porosity (less than 4%) and low permeability (less than 0.5×10−3 μm2). Contrarily, fractures and their dissolution processes along strike-slip fault zone significantly enhance matrix permeability by more than one order of magnitude and matrix porosity by more than one time. Widespread “sweet spot” fracture-vuggy reservoirs are found along the strike-slip fault zone, formed at the end of the Ediacaran. These fractured reservoirs are controlled by the coupling mechanisms of sedimentary microfacies, fracturing and karstification. Karstification prevails at the platform margin, while both fracturing and karstification control high-quality reservoirs in the intraplatform, resulting in reservoir diversity in terms of scale, assemblage and type. The architecture of the strike-slip fault zone governed the differential distribution of fracture zones and the fault-controlled “sweet spot” reservoirs, leading to wide fractured-vuggy reservoirs across the strike-slip fault zone. In conclusion, the intracratonic weak strike-slip fault can play a crucial role in improving tight carbonate reservoir, and the strike-slip fault-related “sweet spot” reservoir emerges as a unique and promising target for the efficient development of deep hydrocarbon resources. Tailored development strategies need to be implemented for these reservoirs, considering the diverse and differential impacts exerted by strike-slip faults on the reservoirs.

Qualitative and quantitative diagenetic modelling in a tight carbonate reservoir in north-western Iraq

The diagenetic history of the Butmah Formation (Lower Jurassic) is very complex and affected by several diagenetic processes that worked effectively with fracturing control to create the final pore network. A microscopic study, core plug and well log analyses were combined in this study in order to describe, and differentiate between, the diagenetic and fracturing control that created the final pore system of the formation. The diagenetic processes of the Butmah Formation were studied in depth to describe the diagenetic stages and identify the elements that may compose a petrodiagenetic pathway illustrating its effect on the reservoir quality of the Butmah Formation. Accordingly, the Butmah Formation samples were divided into three petrophysical fields controlled mainly by fracturing and diagenesis, which were then used to develop a new method for estimating the pre-dolomitisation petrophysical properties of the dolomite samples and the post-dolomitisation petrophysical properties of the limestone samples. Consequently, the output of applying this method allows us to effectively begin to predict each of the elements that may compose a petrodiagenetic pathway for the Butmah Formation and make its reservoir characterisation integrated and more understandable. The new method provided a good prediction of matrix porosity and permeability, as well as allowing the estimation of reservoir properties of any other carbonate reservoir in petroleum development projects when there are no core samples in some formation intervals within boreholes.

Petrophysical Analysis Based on Well Logging Data for Tight Carbonate Reservoir: The SADI Formation Case in Halfaya Oil Field

Carbonate reservoirs are an essential source of hydrocarbons worldwide, and their petrophysical properties play a crucial role in hydrocarbon production. Carbonate reservoirs' most critical petrophysical properties are porosity, permeability, and water saturation. A tight reservoir refers to a reservoir with low porosity and permeability, which means it is difficult for fluids to move from one side to another. This study's primary goal is to evaluate reservoir properties and lithological identification of the SADI Formation in the Halfaya oil field. It is considered one of Iraq's most significant oilfields, 35 km south of Amarah. The Sadi formation consists of four units: A, B1, B2, and B3. Sadi A was excluded as it was not filled with hydrocarbons. The structural and petrophysical models were built based on data gathered from five oil wells. The data from the available well logs, including RHOB, NPHI, SONIC, Gamma-ray, Caliper, and resistivity logs, was used to calculate the petrophysical properties. These logs were analyzed and corrected for environmental factors using IP V3.5 software. where the average formation water resistivity (Rw = 0.04), average mud filtrate resistivity (Rmf = 0.06), and Archie's parameters (m = 2, n = 1.9, and a = 1) were determined. The well-log data values calculated the porosity, permeability, water saturation, and net-to-gross thickness ratio (N/G).

The synergistic effects of cationic surfactant and smart seawater on the recovery of medium-viscosity crude oil from low-permeability carbonates

In the petroleum industry, there is a continuous search for cost-effective enhanced oil recovery (EOR) methods to increase oil extraction. Surfactant-based EOR is one of these effective methods, which is applied after the primary and secondary recovery phases. This paper presents an experimental study on the use of a cationic surfactant, hexadecyltrimethylammonium bromide (CTAB, CH3(CH2)15N(Br)(CH3)3), and smart seawater (SW) for EOR in low-permeability carbonate rock with medium viscosity crude oil. Smart SW was prepared by modifying the concentrations of Ca2+, Mg2+, and SO42- in synthetic offshore SW. The aqueous stability of CTAB was tested in different salinities of smart SW at the reservoir temperature. It was demonstrated that CTAB is stable at 55 °C, which was further confirmed through optical transmittance measurements, revealing values mostly exceeding 93%. The use of smart SW and CTAB significantly altered the wettability of carbonate rock towards water-wet and reduced the interfacial tension (IFT) between crude oil and water, reaching levels below 0.087 mN/m. However, the presence of smart SW increased the adsorption of CTAB onto the surface of carbonate rock. Nevertheless, the adsorption values were significantly below 1 mg/g-rock. Smart SW + CTAB resulted in a total oil recovery of 76.71% of original oil-in-place (OOIP), compared to 71.70% of OOIP with unmodified SW + CTAB. These results suggest that the use of smart SW in combination with CTAB is a promising approach for enhancing the recovery of medium-viscosity oil from tight carbonate reservoirs. Findings from this study can help in better understanding the optimal conditions for the application of smart SW-CTAB flooding to improve the effectiveness of tertiary recovery operations in the upstream oil sector.

Anatomy of fractures: Quantifying fracture geometry utilizing X-ray computed tomography in a chalk-marl reservoir; the Lower Cretaceous Valdemar Field (Danish Central Graben)

Naturally fractured reservoirs tend to show heterogeneous permeability with open fractures enhancing the inherent diagenetic and depositional matrix fabric. Natural open fractures may function as pathways for fluid flow in tight (low matrix permeability) carbonate reservoirs and have a significant impact on the directions, magnitudes and heterogeneities of fluid flow and reservoir rock strength. Open fractures may however also facilitate the invasion of formation water while fractures that are partially or completely cemented can act as barriers to fluid flow. As a result, natural fractures significantly impact reservoir flow rates, anisotropy, storage, connectivity, recovery, and production. X-ray Computed Tomography (CT) offers a non-destructive technique to evaluate petrophysical parameters, such as porosity and permeability, and to assess the geometry, connectivity, and density of natural fractures, which are important constituents of chalk reservoirs. The purpose of this study is to investigate different types of naturally occurring fractures within the Lower Cretaceous succession of the Valdemar Field in the Danish North Sea. Six samples of core displaying at least one natural fracture on the surface are imagined; two samples of open fractures with plumose structure and hackle marks, three samples of shear fractures and finally one sample with deformation bands. The samples are imaged by CT and internal fractures are manually mapped. Image analysis provides detailed information on the fracture geometry in addition it indicates the presence of numerous microfractures hitherto not recorded in the Lower Cretaceous of the Danish North Sea. Core data show that fracture intensity can be significantly greater than calculated by conventional core interpretation techniques based on core surface observations. Further, apertures of both major and minor fractures are measured in CT images of all samples to determine if specific fracture types display characteristic that can distinguish them from other fracture types. As open fracture intensity increases, so too does permeability, likely enhancing reservoir connectivity on the field scale.

Azimuthal Prestack Seismic Anisotropic Inversion on a Deep and Tight Carbonate Reservoir From the North Potwar Basin of Pakistan

Summary Quantitative characterization of deep, tight, and heterogeneous reservoirs plays an important role in identifying hydrocarbon pathways for effective and optimal reservoir field development. In this case study, we used an azimuthal prestack seismic anisotropic inversion approach to estimate attributes of horizontal transverse isotropy (HTI) caused by a set of vertical fractures, oriented cracks, and stress. Anisotropic inversion facilitated the conversion of interface properties to the corresponding layer-based properties, which led to the quantitative interpretation of reservoir properties related to azimuthal variation in seismic amplitudes. To estimate the anisotropy magnitude and the direction of the isotropy axis in HTI media, elastic properties (P-impedance and Vp/Vs) obtained from prestack seismic inversion (using six azimuth × four angle stack) served as inputs. The isotropic low-frequency model (LFM) is used as the foundation of the inversion for all azimuths, and the anisotropy effects are later added by updating the model along the azimuths. The direction of the isotropy plane resulting from the anisotropic inversion is determined by using the maximum horizontal stress as a prior constraint, which eliminates any inherent uncertainty. The workflow used effectively characterized the orientation and density of fractures from the recently discovered oilfield reservoirs of the Paleocene (Lockhart) formation located in Pakistan’s north Potwar Basin. It also helped improve the prediction accuracy for fractures in the study area. According to the observations (fractures) from the exploratory drilled well (D1) in the tight carbonate (Lockhart) reservoir, a significant amount of anisotropy magnitude is observed. This provides the basis for hydrocarbon exploration, field development, and reliable drilling decisions.

Pore structure and fluid mobility of tight carbonate reservoirs in the Western Qaidam Basin, China

AbstractTight carbonate reservoirs in the Western Qaidam Basin have complex lithologies and pore structures. The oil–water mobility law in reservoirs has not yet been completely determined, restricting the formulation of rational reservoir development methods. To bridge this gap, in this study, we used several test methods, such as casting thin sections, mercury intrusion, and nuclear magnetic resonance, to obtain the pore structure and oil–water displacement characteristics of tight carbonate reservoirs in the Western Qaidam Basin. The pore structures of the reservoirs could be categorized into three types: microfractures + dissolved pores + micropores (MFD), microfractures + micropores (MF), and matrix (M). The characteristics of single‐phase oil seepage and water flooding in reservoirs with various pore structures differed evidently. For the MF‐ and M‐types, the water‐locking effect caused by the strong capillary force affected oil charging in the micropores. The effect of the pressure drop on the MFD‐type algal limestone was less than that on the MF‐type limestone (dolomite) because of the occurrence of a non‐Darcy flow. The MFD‐type, which contained microfractures, had preferential seepage channels, resulting in obvious fluid channeling and low water displacement efficiency. Oil−water displacement mainly occurred in the dissolved pores and microfractures, suggesting that starting oil accumulation in the micropores was crucial. This study will assist in efficient development of tight carbonate reservoirs in the Western Qaidam Basin.

Carbonate sedimentation and reservoirs associated with a volcanic mound in an open-marine, deep-water, drowned platform setting, Elaine field area, Upper Cretaceous Anacacho Formation, South Texas U.S.A.

In the Upper Cretaceous Balcones Igneous Province in South and Central Texas, numerous small volcanoes developed during Austin Chalk and Anacacho deposition that are similar to the recent Surtsey volcano offshore of Iceland. The volcanoes formed topographic highs on the deep-water (500–900 ft [150–275 m]), earlier-drowned Comanche Platform. The Elaine field in South Texas is associated with one of these relict volcanic highs and integration of a 3D-seismic dataset and cores permitted a detailed analysis of the deeper-water carbonate depositional systems of the field and associated carbonate reservoir development. This reservoir description extends the understanding of petroleum systems associated with relict, open-marine, deep-water, volcanic highs. In the shallow water at the top of the volcanic topographic highs, a carbonate factory developed that produced coarser-grained carbonate sediments that sourced the deeper-water flanks and toe-of-slope of the mound. Finer-grained carbonate chalk was deposited in the open-marine, deep-water environment adjacent to the volcanic mound. The coarser-grained carbonate sediments sourced from the mound top were transported downslope by gravity-flow processes that formed debrites and hyperconcentrated density-flow deposits. The gravity-flow deposits are composed of a mixture of shallow- and open-marine, deep-water biota indicating they are not shallow-water deposits, but are deeper-water mound-flank and toe-of-slope deposits.In the Anacacho Elaine field, the coarser-grained carbonates (i.e., skeletal packstones and grainstones) associated with the volcanic mound are the oil-producing reservoirs. The Elaine volcanic mound itself is tight and not productive. The Anacacho chalk immediately above the mound are also tight and form the top seal for the Elaine field.Associated with the relict volcanic mounds in the Balcones Igneous Province are a spectrum of carbonate plays. The relict volcano can be a reservoir, but not all the relict volcanic mounds are porous. The coarser-grain carbonates on the top, flanks, and at the base of the volcanic mounds are reservoirs in the area of the Balcones Igneous Province. The chalks deposited penecontemporaneously on the open-marine, deep-water platform can also be tight carbonate reservoirs.

Characteristics of Tight Dolostone Reservoir and Its Main Controlling Factors in the Submember Ma55 of Majiagou Formation in the Western Ordos Basin

The tight carbonate reservoir was controlled by various geological factors, and such factors played different roles in buried depths and formations. Therefore, studies related to the factors controlling carbonate reservoir distribution are of great significance for the prediction and evaluation of high-quality dolostone reservoirs. In this paper, we focus on the controlling factor of the submember Ma55 dolostone reservoir in the western Ordos Basin. The main rock types, reservoir pores, physical properties, and pore structure characteristics of the reservoir were analyzed by thin section identification, physical property analysis, and mercury injection, respectively. Then, the main controlling factors of reservoir development were comprehensively analyzed from the perspectives of palaeostructure, lithofacies palaeogeography, diagenesis, and diagenetic facies. The results show that two kinds of dolostone reservoirs in the submember Ma55 developed in the western Ordos Basin, including intercrystalline pore-type and dissolution pore-type. The former reservoir is primarily characterized by powder-fine dolostone with residual structure, dolomite intercrystalline pore, and micropore with porosity ranging from 2% to 11%. There are three types of pore structures developed in it, such as macropore-medium throat-single peak (MAMS), macropore-fine throat-single peak (MAFS), and medium pore-fine throat-single peak (MEFS). The latter reservoir is mainly featured by powdery crystalline dolostone with gypsum and halite dissolution, moldic pore, and dissolved pore between breccias with a porosity greater than 5%. It consists of two types of pore structures, such as macropore-fine throat-single peak (MAFS) and medium pore-coarse throat-multipeak (MECM). The intercrystalline pore-type dolostone reservoir is mainly controlled by the lithofacies palaeogeographic environment and diagenesis. In specific, the shoal microfacies at the edge of the platform and the active reflux seepage dolomitization are the basic sedimentary environment conditions for reservoir formation and the key to reservoir formation, respectively. The dissolution pore-type dolostone reservoir is primarily influenced by both paleostructure and diagenesis. The relatively high part of the paleostructure provides favorable conditions for the formation of evaporate minerals, and early freshwater dissolution is the key to reservoir formation. This research will provide a theoretical basis for forecasting the favorable distribution areas of different types of dolostone reservoirs.

The effect of fracturing on permeability in carbonate reservoir rocks

Hydrocarbon flow rates from tight carbonate reservoirs often depend on the distribution and properties of fractures throughout the reservoir interval. Hence, quantification of fracture parameters can contribute to understanding of reservoir characteristics and reservoir potential. In this paper, natural fractures in tight carbonate reservoir rocks of the Upper Cretaceous Shiranish Formation in the Taq Taq field (Kurdistan Region of North Iraq) were examined primarily using core samples, XRMI micro-resistivity image logs and conventional wireline logs, with additional dynamic data from well tests and mud logs. The Shiranish Formation is composed of a variety of limestones and argillaceous limestones with arithmetic mean matrix porosity and geometric mean permeability values of 1.64 ± 0.37% and 0.0223 ± 0.59 mD, respectively. However, fracturing has enhanced the permeability by up to four orders of magnitude above the maximum matrix permeability (1.79 ± 0.59 mD). Hydrocarbons are produced entirely from the fracture network in these rock types in the Taq Taq reservoir. Observed open and partially-open fractures have NE-SW orientations; these are classified as macro-fractures with average aperture of 0.17 ± 0.18 mm, arithmetic mean height of 11.70 ± 45.40 mm and arithmetic mean length of 26.7 ± 32.32 cm, where the uncertainties are standard deviations. Fracture aperture size and fracture height control fracture permeability and fluid flow. However, while fracture frequency has no effect on permeability, fracture distance distribution influences flow rates and hydrocarbon production. The dominant NE-SW trending fractures are more likely to remain open within the NE-SW oriented present day stress field at Taq Taq. Furthermore, the effect of overburden stress on the permeability and production rate was examined in laboratory tests on core plugs by increasing the confining stress during permeability measurements using constant flow rates. It was found that the magnitude of fracture permeability was reduced by 70% by applying 4000 psi of confining stress. In the reservoir, this effect would be exacerbated by reduced reservoir fluid pressures as a function of production. However, fractures with apertures greater than 0.10 mm are preserved as productive fractures even at high confining pressures and implying that hydraulic fracturing could open existing fractures significantly, enhancing production. This last observation implies that the use of hydraulic fracturing in tight carbonate reservoirs with natural fracturing is likely to be commercially viable.

Characteristics and origin of the tight grainstone reservoirs of the Yijianfang Formation, middle Ordovician in the Shunnan area, Tarim Basin

Tight carbonate reservoirs are widely distributed in the world and have great economic exploration potential. The Ordovician Yijianfang Formation grainstones in the Shunnan area, Tarim Basin are considered to be hydrocarbon reservoirs. However, there are few visible pores from thin sections and cores, resulting in poor understanding of the pore systems in these tight limestone reservoirs. Core samples collected from 8 wells were prepared to characterize pore networks by combining the optical microscope, scanning electron microscope, helium gas porosimeter, nuclear magnetic resonance and micro-CT analysis. Carbon and oxygen isotopes and fluid inclusion thermometry were adopted to determine the timing of pore-related dissolution. For Yijianfang grainstones, porosity mostly ranges from 0.2 to 3.9%; permeability mostly ranges from 0.01 to 1 mD. Petrographic observations revealed that both intragranular macropores and micropores within grains constitute effective reservoir space. Besides, micropores could account for 50–90% of the total porosity. The intragranular pores only appear in the upper part of the cycle, and the micropores developed in upper and middle of the cycle. There are no pores in the lower part of the cycle. During brief subaerial exposure shortly after deposition, meteoric dissolution created intragranular pores and vugs in grainstones in upper part of the sedimentary cycle. Meanwhile, influenced by the freshwater-involved mineralogical stabilization, micropores transformed from between micrites to between calcite microcrystals within grains in grainstones in the upper and middle part of the cycle. In the burial diagenetic environment, vugs and interparticle pores were destroyed by compaction and burial cements. For packstones in the lower part of the cycle, mineralogical stabilization occurred without freshwater during progressive burial, leading to tight fabric with no or minor micropores. In summary, whether tight limestones can be effective reservoirs depends on whether freshwater is involved in dissolution and mineralogical stabilization. In general, tight grainstone reservoirs of shallowing-upward cycle could provide abundant micropores with few macropores due to the freshwater-involved dissolution and stabilization during subaerial exposure.

A Method for Determination of Rock Fabric Number from Well Logs in Unconventional Tight Oil Carbonates

Summary This paper develops a method for estimation of rock fabric number (RFN) from well logs in unconventional tight oil carbonates with permeability less than 0.1 md. The objective is to investigate the oil potential of a Middle Cretaceous tight carbonate in Mexico. The development of a method for these conditions is challenging as the current approach developed by Lucia (1983) has been explained for carbonates with permeability more than 0.1 md. Core data and drill cuttings available for this study are limited but provide important insights for the log interpretation and for identifying the presence of grainstone, packstone, and wackstone rocks in the unconventional tight carbonate under consideration. A crossplot of RFN vs. rp35 (pore throat radius at 35% cumulative pore volume) permits delimiting intervals with good production potential that are supported by well testing data. Information for the analysis of the Mexican carbonate comes from well logs of nine wells and two re-entry wells, four buildup tests, and a limited amount of core and drill cuttings information. All data were provided by a petroleum company and have been used, for transparency, without any modifications. An unconventional tight carbonate as defined in this paper has a permeability smaller than 0.1 md. The unconventional tight oil carbonate reservoir considered in this study includes 95% of data with permeabilities smaller than 0.1 md and only 5% with permeabilities larger than 0.1 md. The method introduced by Lucia (1983) and Jennings and Lucia (2003) for determining RFN is powerful, but they explained it only for permeabilities larger than 0.1 md, thus the need for a methodology that allows estimating from well logs the presence of grainstone, packstone, and/or wackstone in unconventional tight carbonate reservoirs with permeabilities smaller than 0.1 md. Results indicate that the RFN provides a useful approach for distinguishing grainstone, packstone, and wackstone rocks in unconventional tight carbonate reservoirs. Furthermore, rock fabric can be linked with Pickett plots to provide an integrated quantitative evaluation of RFN, porosity, water saturation, permeability, pore throat radius, and capillary pressure. This integration indicates that there is good oil potential in the Middle Cretaceous unconventional tight carbonate in Mexico.

Comparative analysis of matrix-retarded acidizing methods for tight carbonate reservoirs: Gelled acid, micro-emulsified acid, and foamed acid

Comparative analysis of matrix-retarded acidizing methods for tight carbonate reservoirs: Gelled acid, micro-emulsified acid, and foamed acid

Preliminary hydraulic fracturing campaign strategies for unconventional and tight reservoirs of UAE: Case studies and lessons learned

<abstract> <p>The challenges associated with applying hydraulic fracturing (HF) technology to tight carbonate reservoirs with very low clay content are substantial and demand a unique cost optimization strategy, especially in the context of low oil prices. This study discusses the challenges of applying HF technology to such reservoirs in the UAE. The work presents a comprehensive approach to assess and employ this technology, including a thorough study, a strategic roadmap, screening procedures, a fracturing workflow and strategy and an examination of the distinctive challenges and lessons learned from the process. The primary goal is to formulate a strategy that is applicable to tight and unconventional formations in the UAE, with a strong emphasis on cost optimization. Also, the evaluation methods of the fracturing technologies for these reservoirs were discussed, such as creating valid geomechanical properties to construct a Mechanical Earth Model (MEM) for successful execution and evaluating the reservoir quality. The results showed that conventional acidizing is not effective in stimulating the tight carbonate reservoirs, whereas acid-fracturing has successfully broken down the formation. It was also found that strategic planning, equipment availability, geomechanical studies and building an effective MEM are necessary for obtaining the optimum fracturing design and achieving successful development.</p> </abstract>

Rock physics modelling‐based characterization of deep and tight mixed sedimentary (clastic and carbonate) reservoirs: A case study from North Potwar Basin of Pakistan

AbstractRock physics modelling in deeply buried tight carbonate reservoirs is more challenging than the clastic reservoir because of its complex pore systems. Therefore, modelling complex pore geometries in carbonates reservoir plays an important role in the geophysical measurements to infer pore‐type distribution and their volume fraction, which control the fluid flow properties governed by their complex geological history. This paper presents the use of a robust and practical integrated and iterative seismic petrophysics, and rock physics modelling workflow for the characterization of a deep tight mixed (carbonate and clastic) sedimentary reservoir of a recently discovered oil field located in the Potwar Basin onshore Pakistan. The case study incorporates a careful assessment of well‐log data quality, workflow used for well data conditioning and consequent improvement in data quality, seismic petrophysics and rock physics modelling results. Rock physics diagnostics identified data quality issues and rectified them using appropriate measures to ensure that the rock physics modelling scheme reliably predicts elastic logs honouring different pore types (reference, soft, stiff and cracks) distribution validating from the well observations. Two inclusion‐based rock physics models (Xu and Payne for carbonate and Keys and Xu for clastic) are used to predict rock elastic (P‐sonic, S‐sonic and density) properties responses. Utmost care is exercised to integrate all available data/information, such as Formation Micro‐Imager, mud‐log, testing data and calibrated pore‐type inversion results from well‐log data with the pore geometry revealed by Formation Micro‐Imager and core information. The rock physics modelling provided consistency between elastic and petrophysical properties by calibrating the estimated properties with the measure logs and improved understanding in terms of lithology and fluid separation in the tight carbonate interval. Finally, rock physics‐modelled elastic logs (P‐sonic, S‐sonic and density) are used along with seismic data (partial stacks) and horizons interpretation model to compute pre‐stack synthetic seismograms to calibrate with the preconditioned seismic data. A significant improvement in the well‐to‐seismic calibration is achieved from rock physics modelled logs compared with the original (measured) elastic logs at well location using pre‐stack seismic data, which, further, can help produce reliable reservoir rock properties of interest from the inversion of seismic data.

An intelligent prediction method of fractures in tight carbonate reservoirs

An intelligent prediction method for fractures in tight carbonate reservoir has been established by upgrading single-well fracture identification and interwell fracture trend prediction with artificial intelligence, modifying construction of interwell fracture density model, and modeling fracture network and making fracture property equivalence. This method deeply mines fracture information in multi-source isomerous data of different scales to reduce uncertainties of fracture prediction. Based on conventional fracture indicating parameter method, a prediction method of single-well fractures has been worked out by using 3 kinds of artificial intelligence methods to improve fracture identification accuracy from 3 aspects, small sample classification, multi-scale nonlinear feature extraction, and decreasing variance of the prediction model. Fracture prediction by artificial intelligence using seismic attributes provides many details of inter-well fractures. It is combined with fault-related fracture information predicted by numerical simulation of reservoir geomechanics to improve inter-well fracture trend prediction. An interwell fracture density model for fracture network modeling is built by coupling single-well fracture identification and interwell fracture trend through co-sequential simulation. By taking the tight carbonate reservoir of Oligocene-Miocene AS Formation of A Oilfield in Zagros Basin of the Middle East as an example, the proposed prediction method was applied and verified. The single-well fracture identification improves over 15% compared with the conventional fracture indication parameter method in accuracy rate, and the inter-well fracture prediction improves over 25% compared with the composite seismic attribute prediction. The established fracture network model is well consistent with the fluid production index.

Seismic Description of Deep Strike-Slip Fault Damage Zone by Steerable Pyramid Method in the Sichuan Basin, China

Large quantities of gas resources have been found in the Paleo-Mesozoic carbonate rocks in the Sichuan Basin. However, many wells cannot obtain high production in deep low porosity-permeability reservoirs. For this contribution, we provide a steerable pyramid method for identifying the fault damage zone in the Kaijiang–Liangping platform margin, which is infeasible by conventional seismic methods. The results show that steerable pyramid processing could enhance the seismic fault imaging and a series of NW-trending strike-slip faults are found along the trend of the carbonate platform margin. The steerable pyramid attribute presents distinct vertical and horizontal boundaries of the fault damage zone, and heterogeneous intensity of an un-through-going damage zone. The width of the fault damage zone is generally varied in the range of 100–500 m, and could be increased to more than 1000 m in the fault overlap zone, intersection area, and fault tips. Further, the fault damage zone plays a constructive role in the high gas production in the deep tight carbonate reservoir. The results suggest the steerable pyramid method is favorable for identifying the weak strike-slip faults and their damage zone. The width of the fault damage zone is closely related to fault displacement, and the much wider damage zone is generally influenced by the fault overlapping and interaction. The fractured reservoirs in the fault damage zone could be a new favorable exploitation domain in the Sichuan Basin.

Microbial induced mechano-petrophysical modified properties to improve hydrocarbon recovery in carbonate reservoirs

This study focuses on the impact of a microbial-induced process on mechanical and petrophysical properties of tight carbonate hydrocarbon reservoirs and its implication for hydrocarbon recovery from carbonate reservoirs. Tight carbonate core samples were treated with cultured microbial media of Clostridium acetobutylicum at 42 °C for 14 days. The before- and after-treatment mechanical (unconfined compressive strength, UCS; fracture toughness, Ks) and petrophysical (porosity and permeability) properties of the core samples were investigated and analyzed. The scratch test method was used for assessing the geomechanical properties. The results indicate that in tight carbonate reservoirs, post-treatment microbial-induced alterations could degrade its mechanical integrity with −30% UCS and −28% Ks. The post-treatment improvement in petrophysical properties of the tight carbonate rocks yielded a 355% increase the porosity and ＋734% in permeability. The application of the technology presented in this study can potentially enhance the susceptibility for induced fractures to nucleate and propagate during reservoir stimulation in tight carbonate reservoirs, and enhance flow pathways to improve hydrocarbon recovery. This study provides insight into microbial-induced alteration technology that can be incorporated with other reservoir-stimulation techniques to further enhance hydrocarbon recovery from carbonate reservoirs.

An integrated petrographical, petrophysical and organic geochemical characterization of the Lower Turonian Abu Roash-F carbonates, Abu Gharadig field, Egypt – Inferences on self-sourced unconventional reservoir potential

Marine carbonate source rocks are ubiquitous worldwide and many of these can act as self-sourced unconventional reservoirs. This work focuses on the Lower Turonian Abu Roash-F Member from the prolific Abu Gharadig field, Western Desert, Egypt which consists of argillaceous and bioclastic limestones. Thin section petrographic analysis indicated the presence of various microfacies assemblages dominated by planktonic foraminifera in association with abundant calcispheres indicating an open-marine, deep-shelf depositional setting. The X-Ray Fluorescence data exhibits overall poor detrital influx throughout the AR-F while the Mo and V enrichment in the lower part represents Late Cenomanian/Early Turonian (C/T) oceanic anoxic event (OAE2) associated with the global sea-level rise. Geochemical analysis suggests that the studied AR-F has good to very good total organic carbon (TOC) content (up to 3.57 wt%) consists of Type-II kerogen (HI > 400 mgHC/gTOC), thermally mature (Tmax ∼ 438.5 °C) and presently within the oil generation window. The TOC-rich (>2.5 wt%), highly anoxic lower AR-F showed higher oil production probability which is inferred as a sweet spot zone. Petrophysically AR-F is very tight and impervious with 0.9–2.6% porosity and negligible permeability (<0.0033 mD), dominated by nano- and microporosities. Micritization is found to be the most dominant diagenetic process reducing the AR-F storage capacity, followed by calcite cementation filling the bioclast tests and fractures, chemical compaction (stylolites), and minor dolomitization. 2D CT scanning revealed some minor vuggy or fracture pores but those were isolated and lacks in connectivity, which therefore did not improve the flow capacity of AR-F. Therefore, stimulation is necessary to bring this tight carbonate reservoir into production. This study offers critical insights on the AR-F Member which is an excellent source rock and at the same time holds potential as a promising unconventional reservoir.