Development Of Carbonate Reservoirs Research Articles

Pore-throat structure characteristics and its impact on the porosity and permeability relationship of Carboniferous carbonate reservoirs in eastern edge of Pre-Caspian Basin

Abstract Carboniferous carbonate reservoirs at the eastern edge of the Pre-Caspian Basin have undergone complex sedimentation, diagenesis and tectonism processes, and developed various reservoir space types of pores, cavities and fractures with complicated combination patterns which create intricate pore-throats structure. The complex pore-throat structure leads to the complex porosity-permeability relationship, bringing great challenges for classification and evaluation of reservoirs and efficient development. Based on the comprehensive analysis on cores, thin sections, SEM, mercury intrusion, routine core analysis and various tests, this paper systematically investigated the features and main controlling factors of pore-throats structure and its impact on the porosity-permeability relationship of the four reservoir types which were pore-cavity-fracture, pore-cavity, pore-fracture and pore, and three progresses are made. (1) A set of classification and descriptive approach for pore-throat structure of Carboniferous carbonate reservoirs applied to the eastern edge of the Pre-Caspian Basin was established. Four types of pore-throat structures were developed which were wide multimodal mode, wide bimodal mode, centralized unimodal mode and asymmetry bimodal mode, respectively. The discriminant index of pore-throat structure was proposed, realizing the quantitative characterization of pore-throat structure types. (2) The microscopic heterogeneity of pore reservoir was the strongest and four types of pore-throat structures were all developed. The pore-fracture and pore-cavity-fracture reservoirs took the second place, and the microscopic heterogeneity of pore-cavity reservoir was the weakest. It was revealed that the main controlling factor of pore-throat structure was the combination patterns of reservoir space types formed by sedimentation, diagenesis and tectonism. (3) It was revealed that the development of various pore-throat structure types was the important factor affecting poroperm relationship of reservoirs. The calculation accuracy of permeability of reservoirs can be improved remarkably by subdividing the pore-throat structure types. This study deepens the understanding of pore-throat structure of complicated carbonate reservoirs, and is conducive to classification and evaluation, establishment of precise porosity-permeability relationship and highly efficient development of carbonate reservoirs.

Modification of two-scale continuum model and numerical studies for carbonate matrix acidizing

Matrix acidizing is one of the most effective stimulation techniques to achieve industrial development of carbonate reservoirs. In this work, a two-scale continuum model is modified to consider mass conversion between liquid and solid phases. The results show that the mass conversion between liquid and solid phases is very important and nonnegligible. Compared to modified model, original model overestimates amount of acid required for core breakthrough. A series of numerical cases based on modified model are presented for sensitivity studies on acid injection conditions, core geometries, acid, and rock properties. The modified model can well capture optimum acid injection velocity and typical dissolution patterns observed in experimental studies. Pore volume to breakthrough (PVBT) is very sensitive to acid injection concentration and acid surface reaction rate. Increasing injected acid concentration substantially reduces the amount of solute required but acid injection mass does not change significantly. Strong acid can effectively reduce least pore volume to breakthrough (PVBT) but meanwhile optimum acid injection velocity obviously increases. The simulation results are compared with available experimental and numerical data and found to attain a consistent match.

Hydrocarbon-bearing characteristics of the SB1 strike-slip fault zone in the north of the Shuntuo Low Uplift, Tarim Basin

The SB1 strike-slip fault zone, which developed in the north of the Shuntuo Low Uplift of the Tarim Basin, plays an essential role in reservoir formation and hydrocarbon accumulation in deep Ordovician carbonate rocks. In this research, through the analysis of high-quality 3D seismic volumes, outcrop, drilling and production data, the hydrocarbon-bearing characteristics of the SB1 fault are systematically studied. The SB1 fault developed sequentially in the Paleozoic and formed as a result of a three-fold evolution: Middle Caledonian (phase III), Late Caledonian–Early Hercynian and Middle–Late Hercynian. Multiple fault activities are beneficial to reservoir development and hydrocarbon filling. In the Middle–Lower Ordovician carbonate strata, linear shear structures without deformation segments, pull-apart structure segments and push-up structure segments alternately developed along the SB1 fault. Pull-apart structure segments are the most favourable areas for oil and gas accumulation. The tight fault core in the centre of the strike-slip fault zone is typically a low-permeability barrier, whilst the damage zones on both sides of the fault core are migration pathways and accumulation traps for hydrocarbons, leading to heterogeneity in the reservoirs controlled by the SB1 fault. This study provides a reference for hydrocarbon exploration and development of similar deep-marine carbonate reservoirs controlled by strike-slip faults in the Tarim Basin and similar ancient hydrocarbon-rich basins.

Progress and development direction of technologies for deep marine carbonate gas reservoirs in the Sichuan Basin

In recent years, deep marine carbonate gas reservoirs have become an important field in increasing conventional gas reserves and production in the Sichuan Basin. Due to complex hydrocarbon accumulation conditions, great burial depth and complicate gas–water relation, however, this type of gas reservoirs faces huge challenges to realize efficient development. In this paper, the development history of deep marine carbonate gas reservoirs in the Sichuan Basin was analyzed. Then, based on such development characteristics and practices, special technologies were summarized. Finally, the research direction for the coming years was pointed out. And the following research results were obtained. First, the reserves of the deep marine carbonate gas reservoirs in the Sichuan Basin vary greatly in scales and types. The reservoirs are of poor quality and strong heterogeneity with active edge and bottom water. The raw gas generally contains acidic gas. What's more, most development wells are located in mountainous areas, and drilled formations are complicated in vertical distribution. Second, special technologies have been formed in the Sichuan Basin, including the fine description technology for strong-heterogeneity gas reservoirs in deep low-gentle structures, the optimized water control technology for the water-bearing gas reservoirs with developed small-scale fractures and vugs, drilling, completion and stimulation engineering technologies for deep complex gas wells, and the support technology for clean and safe development of sulfur-bearing gas reservoirs, so as to provide support for the efficient productivity construction and optimized development of giant marine carbonate reservoirs in the Sichuan Basin. Third, as the development of the carbonate gas reservoirs in the Sichuan Basin begins to convert into the gas reservoirs in ultra-deep complex structures, the next direction of technological research is the fine structure description and thin reservoir prediction technique, across-scale numerical simulation technique, drilling and completion and gas production engineering technique for deep marine carbonate gas reservoirs. It is concluded that these special technologies for the efficient development of deep marine carbonate gas reservoirs provides powerful support for PetroChina Southwest Company to build up giant gas fields with the reserves of tens of billions of cubic meters and realize the leaping growth of gas production. The research results can provide valuable technical reference for the development of deep marine carbonate gas reservoirs at home and abroad.

Origin, hydrocarbon accumulation and oil-gas enrichment of fault-karst carbonate reservoirs: A case study of Ordovician carbonate reservoirs in South Tahe area of Halahatang oilfield, Tarim Basin

Abstract Based on comprehensive analysis of tectonic and fault evolution, core, well logging, seismic, drilling, and production data, the reservoir space characteristic, distribution, origin of fault-karst carbonate reservoir in Yueman block of South Tahe area, Halahatang oilfield, Tarim Basin, were studied systematically. And the regular pattern of hydrocarbon accumulation and enrichment was analyzed systematically based on development practice of the reservoirs. The results show that fault-karst carbonate reservoirs are distributed in the form of “body by body” discontinuously, heterogeneously and irregularly, which are controlled by the development of faults. Three formation models of fault-karst carbonate reservoirs, namely, the models controlled by the main deep-large fault, the secondary fault and the secondary internal fault, are built. The hydrocarbon accumulation and enrichment of fault-karst carbonate reservoirs is controlled by the spatiotemporal matching relation between hydrocarbon generation period and fault activity, and the size and segmentation of fault. The study results can effectively guide the well deployment and help the efficient development of fault-karst carbonate reservoirs of South Tahe area, Halahatang oilfield.

An extended finite element method for the prediction of acid-etched fracture propagation behavior in fractured-vuggy carbonate reservoirs

Acid fracturing is an important method of achieving high-efficiency economic development of carbonate reservoirs. The propagation of acid-etched fractures is affected significantly by geological discontinuities such as natural fractures and limestone caverns. Therefore, acid-etched fractures in carbonate reservoirs also differ greatly in morphology to those inhomogeneous reservoirs. However, no clear propagation mechanism has been available to describe and predict the propagation behavior of acid-etched fractures. In order to investigate the process, based on the extended finite element method, a set of acid-etched fracture propagation models for fracture-vuggy reservoirs, including mechanical, hydro and acid-rock reaction modules, were established. The effect of vuggy, natural fracture and acid on the initiation and propagation of induced fracture, the surface morphology of the acid-etched fractures, the effects of the number of perforation clusters, and natural limestone caverns, were considered. The calculated geometric size of the fracture was compared with the results based on the CGD (Christianovich-Geertsma-Daneshy model) model, for validating the reliability of the present model. The effects of the injection fracturing liquid type and the number of perforation clusters on fracture initiation position, propagation path, and surface morphology were evaluated. The calculation results also were compared with large-scale tri-axial physical experiment data. Finally, the fracture propagation behavior in rock samples containing natural fractures and limestone caverns was simulated in order to examine examining the effects of the limestone caverns with different sizes at different positions on the propagation path of acid-etched fractures. The interaction between induced fractures and limestone caverns was discussed. The induced fractures priority was given to activating natural fractures around the wellbore and extending along the natural fractures and tended to be deflected from the maximum horizontal principal stress direction due to the existence of caverns arrested fractures to a certain degree.

Investigation on the micro-flow mechanism of enhanced oil recovery by low-salinity water flooding in carbonate reservoir

Low-salinity water flooding has become a significant way to develop carbonate reservoirs in recent years. Clarifying the micro-flow mechanism of low-salinity water flooding (LSWF) on enhanced oil recovery (EOR) becomes the focus of this issue. In this paper, firstly, we conducted a series of experiments to prove that LSWF can improve the recovery of carbonate reservoirs. And then, a new relative permeability model and water content calculation model are established considering interface micro-forces and capillary pressure. The result shows that the present model has good agreement with the experimental results, which can characterize the micro-displacement mechanism of LSWF. The recovery rate can be increased by 9% compared with the formation water flooding (FWF). Under low-salinity conditions, the wettability of rock surface is changed by the chemical reaction of divalent ions (Ca2+, Mg2+, SO42−), which shows more hydrophilic. And the optimal salinity of the brine range from 1400 mg/L to 3200 mg/L. Besides, LSWF can reduce the residual oil saturation, which is the macro cause for EOR. This study has provided a quick and reasonable guide to know the micro-flow mechanism of LSWF and theoretical basis for development of carbonate reservoirs by LSWF.

Evaluation and Utilization of Nano-Micron Polymer Plug for Heterogeneous Carbonate Reservoir with Thief Zones

Thief zones are highly permeable zones in oil reservoirs which affect the performance greatly during water flooding. The most effective way to prevent the invalid circle of thief layer is the injection of high-concentration polymer slugs. In this paper, the plugging effect of nano-micron polymer flooding on the thief zone is studied by experimental and theoretical analysis. The changes of water content and flow resistance were analyzed under different conditions. The result shows that the model presented here has good agreement with the experimental results. The displacement effect is the best when the thief zone is located on the upper part of the reservoir. And the water content will decrease with the increase of flow resistance after nano-micron polymer injection. Besides, the higher the polymer concentration, the more obvious the decrease of water content, and more effective the plugging. This study has provided a quick and reasonable guide in the later adjustment of water flooding development of carbonate reservoirs with thief layers.

Changes in pore structures and porosity-permeability evolution of saline-lacustrine carbonate reservoir triggered by fresh water-rock reaction

Salt dissolution occurs frequently during the drilling, water-flooding development and other operations of saline-lacustrine carbonate reservoirs, greatly changing the petrophysical properties of rocks. To understand the salt dissolution behavior and its effect on the rock structures, core-flooding and crushed sample soaking experiments are conducted on saline-lacustrine carbonate samples. Results indicate that ion concentration from core flooding experiments increases first and then decreases fast with time proceeding, while it has a positive e-based logarithmic relation with soaking time from sample soaking experiments. According to ion analysis, the main dissolved salt minerals include halite, anhydrite and glauberite. With the size increase of crushed samples, the dissolved mass is decreased, having a positive logarithmic relation with sample specific surface, and sample specific surface is logarithmically increased with soaking time. Rock capillary pressure curves get lower, and pore throat size distributions become wider than those before dissolution of salt minerals. Salt dissolution mainly affects the pore microstructure with size ranging from 0.01 to 10 μm, while having little effect on nanopores <0.01 μm. Salt dissolution could make the clay swelling and fine migration, inhibiting the increment in porosity and permeability. In addition, rock permeability has a power relation with porosity during the dissolution, and a permeability-growth-controlling threshold porosity is observed, which indicates the transformation point of dissolution area from pore bodies to throats. Furthermore, a mathematical model for characterization of rock porosity and permeability evolution relationship is derived, which allows for rock microstructure, salt composition and dissolution, and clay composition and swelling behavior.

Carbonate reservoir development in the Canning Basin, Western Australia

We present a multidisciplinary study investigating depositional and diagenetic impacts on pore system variability in carbonate systems from the Canning Basin. With some of Australia’s only known co...

Fracture and fluid flow paths analysis of an offshore carbonate reservoir using oil-based mud images and petrophysical logs

Abstract Hydrocarbon production and development of carbonate reservoirs mainly depend on fractures that cross wells. Therefore, quantification of their characteristics has a crucial role in reservoir management. In this research, bedding planes, faults and different types of fractures of a Cenomanian-Turonian carbonate reservoir in the Persian Gulf were studied. Image logs including the oil-based mud image (OBMI) and the ultrasonic borehole image (UBI) were used. The results from conventional petrophysical log suites were compared to the image logs results. Based on the image logs, structural characteristics of bedding planes were reconstructed by estimating their strike, dip, dip direction and layer thickness. Fracture types and their distribution and the geometry in the well were determined. Also, fault zone geometry and type of faults were evaluated. The results indicate that majority of the bedding planes have a N58W strike with an average dip of 18° towards NE. Furthermore, both tensile and shear fractures were distinguished, although shear fractures dominated and the 328/36SW and 29/45NW were found to be the prevalent orientation of fractures. Based on petrophysical logs analyses, four facies groups are distinguished and two statistical relationships are proposed for estimation of discontinuities. The proposed index for evaluation of fractures using petrophysical logs shows good performance and it could be used for wells when no image log is available. Especially in offshore reservoirs, lack of outcrop and unavailability of image logs of old wells lead to unknown fractures characteristics which will be addressed by the suggested equations.

Practice and knowledge of volumetric development of deep fractured-vuggy carbonate reservoirs in Tarim Basin, NW China

Abstract Different from the continental layered sandstone and fracture-pore carbonate reservoirs, the fractured-vuggy carbonate reservoirs in the Tarim Basin are mainly composed of fractured-vuggy bodies of different sizes and shapes. Based on years of study on the geological features, flow mechanisms, high-precision depiction and the recovery mode of fractured-vuggy bodies, the idea of “volumetric development” is proposed and put into practice. A “body by body” production methodology is established with respect to volumetric unit of fractures and vugs based on vuggy body's spatial allocation and reserves. A variety of development wells, various technological methods, and multi-type injection media are used to develop this type of reservoirs in an all-around way. As a result, the resource and production structures of the Tahe oilfield are significantly improved and a highly efficient development is achieved.

Characterization and discrimination of paleokarst breccias and pseudobreccias in carbonate rocks: Insight from Ordovician strata in the northern Tarim Basin, China

Ordovician carbonates in the central and southern Halahatang area of the Tarim Basin host highly productive hydrocarbon reservoirs. Previous studies suggest that the Yingshan and Yijianfang Formations (the primary producing intervals) have been extensively karstified. However, the current author has not observed significant karst features in the formations based on the detailed study of 380 m long cores from 16 wells. In the current study, true breccias are only identified in two wells in the Lianglitage Formation, including crackle and mosaic breccias as well as chaotic, clast-supported breccias. The breccias are interpreted as karst-related breccias that formed in near-surface environments during pre-Silurian subaerial exposure, and timing of karstification probably coincided with eustatic sea-level fall caused by the middle Katian glaciation. Widespread “karst features” of the Yijianfang and Yingshan Formations identified as breccias or sediment-filled cavities in many previous studies are actually only a mottled fabric (pseudobreccia) caused by selective diagenetic alteration and infiltration of bitumen in the deeper subsurface. The color difference between pseudoclasts and pseudomatrix does not reflect a mixture of clasts and cave sediments, but instead is characterized by distinct, nevertheless commonly subtle, variations in the limestone texture and whether bitumen is present. One the basis of core and thin section observations, SEM (scanning electron microscope) and XRD (X-ray diffraction) analyses, the author presents criteria to distinguish karst breccias and pseudobreccias that can be applied to other similar carbonate deposits. Discrimination between karst breccias and nodular limestones is also presented. Differentiation of karst and pseudokarst features serves not only an academic interest, but also has important applications for the exploration and development of carbonate reservoirs.

Diagenetic evolution characteristics of Paleogene lacustrine carbonate reservoirs in central Bohai sea, China

The distribution of high-quality reservoirs in Shahejie Formation of the Shijiutuo Uplift, Central Bohai Sea, is the key factor to determine the oil and gas accumulation. Likewise, diagenesis is the key factor to control the development of lacustrine carbonate reservoirs in Shahejie Formation. So, accurately analyzing the diagenesis is very important for improving reservoir prediction. By means of polarizing microscopy, fluorescence microscopy, scanning electron microscopy, cathodoluminescence and electron probe analysis techniques, reservoir characteristics and diagenetic evolution sequence are determined. The diagenetic stage was determined by X-ray diffraction of clay minerals. Gas-liquid inclusion homogenization temperature and oxygen isotope geothermometer are used to analyze the formation period of authigenic carbonate minerals. Bioclastic dolostone is the main type of carbonate rocks, followed by dolarenite. The visceral cavity pores and residual primary intergranular pores are the main reservoir space. The diagenetic evolution sequence is established based on the different occurrences of multistage dolomites and their relationship with other diagenesis phenomena, and the final diagenetic stage is the phase B of mesodiagenesis. The favorable diagenesis for reservoir physical properties is the early formation of the pore-lining dolomite deposited in the eodiagenesis and the grain-coating dolomite deposited in the syndiagenesis, both of which weaken the subsequent mechanical compaction to the reservoir porosity.

Optimum development options and strategies for water injection development of carbonate reservoirs in the Middle East

Abstract Through the research on several carbonate reservoirs developed in the Middle East, the basic characteristics of different types of carbonate reservoirs are determined, and a set of high-efficiency water injection development options and strategies are presented. Hidden baffles and barriers exist in carbonate reservoirs in the Middle East, so the reservoirs could be divided into different separated development units based on the baffles and barriers characteristics. Flexible and diverse profile control techniques such as high angle wells and simple and applicative zonal water injection have been introduced to improve the control and development degree of reservoirs. Three principal water injection development methods suitable for different carbonate reservoirs in the Middle East are proposed, including the combination of crestal gas injection and peripheral water injection, bottom interval injection and top interval production (buoyancy underpinning), and “weak point and strong plane” area well pattern. Based on the characteristics of very low shale content, fast and far pressure transmission in the Middle East carbonate reservoirs, a large well-spacing flood pattern is recommended, and reasonable development strategies have been made such as moderate water injection rate and maintaining reasonable production pressure drawdown and voidage replacement ratio, so as to maximize the recovery of reservoirs in the none or low water cut period.

Theories and practices of carbonate reservoirs development in China

Abstract Carbonate reservoirs in China have the characteristics of diversified accumulation pattern, complex structure and varying reservoir conditions. Concerning these characteristics, this article tracks the technical breakthroughs and related practices since the 1950s, summarizes the developed theory and technologies of carbonate reservoir development, analyzes their adaptability and problems, and proposes their development trend. The following theory and technologies have come into being: (1) carbonate reservoir formation mechanisms and compound flow mechanisms in complex medium; (2) reservoir identification and description technologies based on geophysics and discrete fracture-vuggy modeling method; (3) well testing analysis technology and numerical simulation method of coupling free flow and porous media flow; (4) enhanced oil recovery techniques for nitrogen single well huff and puff, and water flooding development techniques with well pattern design in spatial structure, changed intensity water injection, water plugging and channel blocking as the core; (5) drilling and completion techniques, acid fracturing techniques and its production increasing techniques. To realize the efficient development of carbonate oil and gas reservoirs, researches in four aspects need to be done: (1) complex reservoir description technology with higher accuracy; (2) various enhanced oil recovery techniques; (3) improving the drilling method and acid fracturing method for ultra-deep carbonate reservoir and significantly cutting engineering cost; (4) strengthening the technological integration of information, big data, cloud computation, and artificial intelligence in oilfield development to realize the smart development of oilfield.

The sedimentary control factors for the cretaceous Mishrif and the tertiary Asmari carbonate reservoirs in southern east Iraq

There are three essential geological factors which interact and restrict each other’s: sedimentation, diagenesis and tectonism, that control the development and evolution of carbonate reservoirs. Regard to karst carbonate reservoir, karstification plays a key role in the formation of carbonate reservoirs by the formation of dissolution pore system; The tectonic movement exposes or upthrow the strata and provides the conditions for karstification. The fractures generated by them can overwhelmingly improve the seepage capacity of carbonate rocks and promote the occurrence of karstification. This paper mainly discuss how sedimentary factors control the development and evolution of carbonate reservoirs in cretaceous Mishrif and tertiary Asmari formations in east southern Iraq.

Концептуальное геологическое моделирование как основа разработки карбонатных коллекторов на примере месторождения Ближнего Востока

Концептуальное геологическое моделирование как основа разработки карбонатных коллекторов на примере месторождения Ближнего Востока

Study and Application of Hydraulic Jet Radial Drilling in Carbonate Reservoirs

The hydraulic jet radial drilling technology is a low-cost production and completion technology for effectively developing low-permeability marginal reservoirs, thin-bed reservoirs with edge and bottom water, and depleted oil and gas fields. It has been successfully and widely applied in sandstone, coalbed methane, and heavy oil reservoirs. In order to explore its application in carbonate reservoirs, specially economically and efficiently develop low-permeability thin-layer carbonate reservoirs, the pilot of hydraulic jet radial drilling technology in the carbonate reservoir application was successfully completed through the research on the principle, advantage, existing problems and application condition of radial jet drilling, economic productivity was got, and cost-effective development was achieved, this pilot provided a method for the development of low permeability thin layer carbonate reservoir. This research shows that the hydraulic jet radial drilling technology has its unique technical advantages, which can enable some reservoirs to increase production or injection, but it is not suitable for every well, it has certain applicable conditions, such as the depth, thickness, and temperature of the target zone, and the conditions of well inclination, casing and wellbore have certain requirements too. The hydraulic jet radial drilling technology can achieve the expected target based on the zone and well selection, in selecting layers, as far as possible, homogeneous oil layers with good reservoir physical properties should be selected; the key of its successful implementation is the depth and azimuth control, wellbore diameter and operation safety control. The application of hydraulic jet radial drilling in carbonate reservoirs will make the spray hose wear large due to its high Young's modulus the high hardness, which affects the treatment footage; the use of acid in jet drilling can increase the radial drilling footage, but on the other hand can also produce impurities affecting the operation. The working fluid for jetting must meet the requirements of no solid phase.

Challenges and Methods to Improve Well Cementing Quality for 177.8 mm Liner in Gaoshiti-Moxi Area

Gaoshiti-Moxi structure belt of An-Yue Sinian gas reservoir, which was China’s largest monomer Marine carbonate gas reservoir up to now, located in vying-dragon female temple structure group that belong to the ancient uplift slope of the middle of Sichuan. With the exploration and development of high temperature and high pressure carbonate reservoir, a large number of challenges and problems, such as long isolation section, active oil-gas show, large temperature difference, prone to super retarding cement slurry and gas channeling at flare position, have been encountered in the cementing of 177.8 mm hang-liner. In order to solve these problems, numerous measures and methods have been put into use for reducing the safety risk of cementing and improving cementing quality. The large temperature difference channeling cement slurry system, effective anti-pollution spacer and high-pressure packer type liner hanger were developed and applied for field tests in the early stage of development. In addition, equilibrium pressure cementing technology, optimizing of centralizer placement and plasma column structure, improvement of pump displacement and hold pressure while waiting on cement were used to ensure nice displacement efficiency and cementing quality. As Moxi X well for example, the cementing quality factor of merit was 40.29% and the qualification rate was up to 78.87% after adopting the cementing measures and methods above. The cementing quality was much higher than previous level and provided technical support for Gaoshiti-Moxi area.