Movable Water Saturation Research Articles

Predicting the compressive strength of tight sandstone based on the low field NMR and pseudo-triaxial compression measurements

The compressive strength is very important for petroleum and other engineering studies. However, the effect of pore size and fluid distribution on the rock’s strength is not fully understood. We developed comprehensive research to study the controlling factors of the compressive strength based on low field nuclear magnetic resonance (NMR) measurements and pseudo-triaxial compression test for tight sandstones. The relationship between the compressive strength and the NMR obtained parameters are investigated completely, aiming for a better estimation of the compressive strength using the NMR data. The result shows that the rock’s strength is strongly controlled by the pore size distribution and the fluid existing state. Generally, the compressive strength is negatively correlated with the average transversal relaxation time, the movable water saturation, and the porosity, but positively correlated with the irreducible water saturation. The result reveals that the rock with larger pore radius and higher percentage of movable fluid is easier to reach the failure state. Further, the precision of the empirical model by multiple regression of the geometric mean of the relaxation time and the porosity is greatly improved compared with the model established by the brittle minerals, which is potentially to be use for geophysical prospecting when the NMR logging data is available.

Research on Transformation of Connate Water to Movable Water in Water-Bearing Tight Gas Reservoirs

The Dongsheng gas field is a water-bearing tight gas reservoir characterized by high connate water saturation. During gas production, the transformation of connate water into movable water introduces a unique water production mode, significantly impacting gas reservoir recovery. Current experimental and theoretical methods for assessing formation water mobility are static and do not address the transformation mechanism from connate into movable water. In this study, we considered dynamic changes in formation stress and proposed the mechanism for the transformation of connate water into movable water during depressurization, involving the expansion of connate water films and the reduction of pore volume. We developed a novel methodology to calculate the dynamic changes in movable and connate water saturation in tight reservoirs due to reservoir pressure reduction. Furthermore, we quantitatively evaluated the transformation of connate water into movable water in the Dongsheng gas field through laboratory experiments (including formation water expansion tests, connate water tests, and porosity stress sensitivity tests) and theoretical calculations. Results show that under original stress, the initial connate water saturation in the Dongsheng gas field ranges from 50.09% to 58.5%. As reservoir pressure decreases, the maximum increase in movable water saturation ranges from 6.1% to 8.4% due to the transformation of connate water into movable water. This explains why formation water is produced in large quantities during gas production. Therefore, considering the transition of connate water to movable water is crucial when evaluating water production risk. These findings offer valuable guidance for selecting optimal well locations and development layers to reduce reservoir water production risks.

Geochemical characteristics of the braided river reservoir in block 19 of the sulige gas field

The sand body structure and geochemical characteristics of braided river reservoirs are the key geological factors affecting gas production and development effects. The Sulige gas field in the Ordos Basin is an important large-scale gas-producing layer. Owing to the control of sedimentary facies, the geological structure of the sand body changes greatly and its connectivity is poor. The geological characteristics have not yet been elucidated, and this is an important problem restricting the development of the Sulige Gas Field. To solve this problem, this study focuses on the braided river reservoir of the Shihezi Formation in Block 19 of the Sulige Gas Field, conducts geological surveys in the study area, analyzes the geological and geochemical characteristics of the reservoir, and obtains samples through drilling. Through a thin-section test, gas-water two-phase experiment, and simulation test, the braided river reservoir configuration and pore and gas-water characteristics are obtained. The results show that the reservoir lithology in the study area is mainly composed of quartz sandstone, lithic sandstone, and quartzy lithic sandstone, with a porosity of 3%–13% and a permeability of (0.05–0.7) × 10−3 m2. The reservoir has low porosity and low permeability. After drilling samples were obtained, 32 thin-section rock samples were selected. The pore types of the block reservoir mainly (82.9%) consisted of intragranular and intergranular dissolved pores. The difference in pore structure was mainly reflected by the size and distribution of the throat. The distribution of physical properties was 6%–10%, the gas saturation was 61%, the NMR effective porosity was 7.49%, the permeability was 4.08 × 102 μm2, and the physical properties were relatively good. In terms of the study area, the average thickness of the single braided channel in the lower section of He 8 was 4.7 m, the average width of the channel was 963 m, and the composite channel was distributed in a potato shape, parallel to the direction of the main flow. The average length of the channel was 2,147 m and the average width was 844 m. As the porosity increased, the efficiency of gas-driven water also increased, and there was a linear positive correlation between porosity and gas-driven water efficiency. With the increase in movable water saturation, the water-air ratio became larger and water production was greater. In low-amplitude structures and under low-permeability background conditions, for reservoirs with good local pore structure and physical properties, the water remaining at the bottom of the reservoir or sand body was controlled by the accumulation conditions or the weak structural differentiation after accumulation. In terms of the gas and water produced simultaneously in the study area, gas production was less than 2 × 104 m3/d and water production was relatively large at more than 10 m3/d; gas and water were mainly distributed in the downdip part of the main channel structure or in the island lens-shaped permeable sand bodies trapped by the surrounding tight layers. The study results provide theoretical data support for the exploration and production of the Sulige Gas Field.

Rock Fabric of Tight Sandstone and Its Influence on Irreducible Water Saturation in Eastern Ordos Basin

After flowback, the residual fracturing fluid will reduce the gas seepage space and influence natural gas production, which attracts widespread attention. In this study, the irreducible water saturation was investigated, and its controlling factors were clarified. We target the Upper Paleozoic Taiyuan and Shihezi Formations, which belong to a tight gas reservoir in the eastern Ordos Basin. The main experiments include porosity, permeability, mineral composition, nitrogen adsorption, mercury intrusion porosimetry, nuclear magnetic resonance, and high-speed centrifugation. The specific surface area is very low and varies from 0.95 to 4.03 m2/g, and the median pore-throat diameter ranges from 28.6 to 698.6 nm. Through the T2 cutoff value, the water saturation can be divided into movable water saturation (Smov) and irreducible water saturation (Sirr). Furthermore, the Sirr can be divided into water saturation in large pores controlled by the small throat (Sirrl) and water saturation controlled by the capillary force (Sirrc). In both formations, the Sirr has a negative relationship with porosity, permeability, and average pore diameter and exhibits a positive relationship with the specific surface area. The Sirrl has a positive relationship with the median pore-throat diameter in Taiyuan Formation, but the Sirrl has a weak relationship with the median pore-throat diameter in Shihezi Formation. The Sirrc has a negative relationship with porosity, permeability, and average pore diameter and displays a positive relationship with the specific surface area in Taiyuan Formation, but the Sirrc has a weak relationship with these parameters in Shihezi Formation. The relationship difference between Taiyuan and Shihezi Formations was mainly caused by the pore structure, demonstrated by the amplitude ratio in three peaks. Based on the above analysis, this study is conducive to understanding the mechanism of water occurrence and its controlling factors.

Insight into Water Occurrence and Pore Size Distribution by Nuclear Magnetic Resonance in Marine Shale Reservoirs, Southern China

The exploitation of shale gas has been a hot topic, and the understanding of water occurrence and pore size distribution in shale reservoirs plays a crucial role in efficiently developing this type of resource. In this study, we target moderate-shallow marine shales from the Lower Cambrian Niutitang Formation and deep marine shales from the Lower Silurian Longmaxi Formation. Low-field nuclear magnetic resonance is applied to investigate the water occurrence and pore size distribution together with complementary experiments including mineral component analysis, scanning electron microscopy (SEM), contact angle measurement, and high-speed centrifugation. The SEM analysis shows that the moderate-shallow shales have some inorganic nanopores and cracks but almost no organic pores, while the deep shales have abundant organic nanopores. The average movable water saturation of moderate-shallow and deep shales is only 15.5% and 15.2%, respectively. It implies that the high irreducible water saturation is a typical characteristic for both marine shales, which is ascribed to the strong capillary force of the nanopore inhibiting water flowing out of the pore space. By means of high-speed centrifugation and contact angle measurement, the converting factor of the T2 value into pore size was obtained. The average pore size in moderate-shallow shales is 19.4, 21.8, and 26.8 nm, respectively, while the average pore size in deep shales is 109, 57, and 68.4 nm, respectively, for the tested samples. This indicates that organic pores in deep shales contribute to the development of abundant nanopores. This study brings insight into characterizing the water occurrence and pore size distribution of moderate-shallow and deep marine shales.

Micro-Scale Pore-Throat Heterogeneity of Tight Oil Sandstone Reservoirs and Its Influence on Fluid Occurrence State

Quantitatively characterizing the micro-scale heterogeneity of pore throats in tight sandstone reservoirs is the key to accurately describing the influence of pore structures on fluid occurrence characteristics. In this study, taking the Chang 6 Member of the Yanchang Formation in the Huaqing area of the Ordos Basin as an example, the pore-throat heterogeneity of tight sandstone reservoirs and its influence on the fluid occurrence state have been systematically studied using cast thin section, scanning electron microscope, X-ray diffraction, constant velocity mercury intrusion, and nuclear magnetic resonance tests. The main types of pores developed in the target layer were intergranular pores, followed by feldspar dissolution pores. The radius distribution of the intergranular pores is between 5.0 and 210 μm, with an average value of 50.27 μm. In addition, the pore combination types with the best petrophysical properties are the intergranular pore type, the intergranular-dissolution pore type, and the dissolution-intergranular pore type; the average permeability and porosity are 0.62 mD, 0.40 mD, 0.44 mD, and 12.0, 12.3, 12.3%, respectively. The target sandstones contain four typical T2 relaxation time types. The large-pore-fine-throat combination reservoir has the best petrophysical properties. The larger the pore-throat uniformity value, the more uniform the pore-throat radius, and the greater the reservoir permeability. Therefore, the uniformity of throat development controls the seepage capacity of the tight reservoirs. The movable fluid saturation of different pore types has obvious differences. The movable fluid saturations at the 0.1 and 0.5 μm pore diameters of the macro-pore-fine-throat and macro-pore-micro-throat reservoirs both show an obvious inflection point, and the movable water saturation is higher with a larger throat radius.

The Influence of Movable Water on the Gas-Phase Threshold Pressure Gradient in Tight Gas Reservoirs

Threshold pressure gradient (TPG) is a key parameter determining the pore-scale fluid dynamics. In tight gas reservoirs, both gas and water exist in the porous rock, and the existing water can be divided into irreducible and movable water. However, how movable water saturation will influence TPG has not yet been investigated. Therefore herein, nuclear magnetic resonance (NMR) and high-pressure mercury intrusion (HPMI) experiments were performed to determine pore-scale water distribution, movable water saturation, and pore throat distribution in the core plugs. Subsequently, the air bubble method was used to measure TPG as a function of movable water saturation and permeability inside tight gas core plugs, finding that TPG increased from 0.01 MPa/m to 0.25 MPa/m with the movable saturation increased from 2% to 35%. Finally, a semi-empirical model was derived to describe the correlation between TPG, movable water saturation, and permeability, which performed better than previous models in the literature. These insights will advance the fundamental understanding of TPG in tight gas reservoirs and provide useful guidance on tight gas reservoirs development.

The Effect of Fracturing Fluid Saturation on Natural Gas Flow Behavior in Tight Reservoirs

Hydraulic fracturing becomes an essential method to develop tight gas. Under high injection pressure, fracturing fluid entering into the formation will reduce the flow channel. To investigate the influence of water saturation on gas flow behavior, this study conducted the gas relative permeability with water saturation and the flow rate with the pressure gradient at different water saturations. As the two dominant tight gas-bearing intervals, the Upper Paleozoic Taiyuan and Shihezi Formations deposited in Ordos Basin were selected because they are the target layers for holding vast tight gas. Median pore radius in the Taiyuan Formation is higher than the one in the Shihezi Formation, while the most probable seepage pore radius in the Taiyuan Formation is lower than the one in the Shihezi Formation. The average irreducible water saturation is 54.4% in the Taiyuan Formation and 61.6% in the Shihezi Formation, which indicates that the Taiyuan Formation has more movable water. The average critical gas saturation is 80.4% and 69.9% in these two formations, respectively, which indicates that the Shihezi Formation has more movable gas. Both critical gas saturation and irreducible water saturation have a negative relationship with porosity as well as permeability. At the same water saturation, the threshold gradient pressure of the Taiyuan Formation is higher than the one in the Shihezi Formation, which means that water saturation has a great influence on the Taiyuan Formation. Overall, compared with the Shihezi Formation, the Taiyuan Formation has a higher median pore size and movable water saturation, but water saturation has more influence on its gas flow capacity. Our research is conducive to understanding the effect of fracturing fluid filtration on the production of natural gas from tight reservoirs.

Study of the Effect of Movable Water Saturation on Gas Production in Tight Sandstone Gas Reservoirs

The movable water saturation of tight sandstone reservoirs is an important parameter in characterizing water production capacity, and there is a great need to understand the relationship between movable water saturation and water production characteristics. However, movable water behavior in this context remains unclear. In this study, four groups of tight sandstone cores from the Sulige gas field are measured to understand the movable water saturation characteristics. Then, the effects such as reservoir micropore throat, clay mineral and physical properties on movable water saturation are analyzed, and the movable water saturation and water production characteristics are discussed. The results show that higher movable water saturation will result in a greater amount of water in the gas drive. There is a critical pressure difference of the gas drive, and a large amount of movable water will flow out. Movable water saturation is independent of the porosity, permeability and initial water saturation, while it is closely related to the reservoir micropore throat and clay mineral content. Movable water is mainly distributed in the medium and large pores; the larger the proportion of such pores, the higher the degree of movable water saturation. A lower mineral content will lead to higher movable water saturation in tight sandstone gas reservoirs. These results provide clues for identifying gas–water bearing reservoirs and evaluating and predicting the water production characteristics in gas wells in tight sandstone gas reservoirs.

Experimental studies on the movable-water saturations of different-scale pores and relative permeability of low-medium rank coals from the Southern Junggar Basin

The movable-water saturation (MWS) of coal is critical for the flow of coalbed methane (CBM), however, there is a relative lack of research on it, especially of different-scale pores. The relative permeability of coal is another crucial parameter for CBM development due to its strong control on the characteristics of both gas and water production, but it also remains relatively unexplored. Naturally, little attention has been paid to the connections between the MWS and the relative permeability.In this study, an innovative experiment combining water-saturated coal core displacement test with low-field nuclear magnetic resonance (LNMR) was designed to obtain the MWSs of different-scale pores and gas-water relative permeabilities of coals simultaneously. The relative permeability results show that low-medium coals from the Southern Junggar Basin (SJB) were characterized by low MWSs, narrow spans of two-phase flow, and low end-point relative permeabilities to water (Krw*) and gas (Krg*), which were all disadvantageous for CBM development. The calculated MWSs of different-scale pores through the amplitude of T2 spectrum show that “macropores and fractures (MF)” have the highest MWS, with mesopores following next, “micropores and transition pores (MT)” possess the lowest MWS. Correlation analyses show that the total MWSs and movable porosities both present strong positive correlations with the Ka, Krw*, and Krg*. The irreducible water causes a greater obstacle to gas flow than to water flow. The MWSs of different-scale pores all exhibit positive impacts on the coal permeabilities. However, compared with MWSs of MT and MF, the MWSs of mesopores exhibit the strongest correlation with Ka, Krw*, and Krg*, indicating the critical role in affecting the fluid flow of coal. Besides, two significant parameters, cleat tortuosity (η) and cleat size distribution index (λ) of SJB coals, were determined. η and λ were rank-dependent. Coal macerals also have impacts on η and λ.

Analysis of movable fluid distribution in a clastic rock reservoir using nuclear magnetic resonance and computed tomography

Studying influence factors of movable fluid distribution is of significance for the development of clastic reservoirs. In this paper, the characteristics of movable fluid distribution in clastic core samples from Tarim Oilfield were studied with nuclear magnetic resonance (NMR) tests and computed tomography (CT) scanning. The movable fluid saturations of each core sample under centrifugal pressure of 3 psi, 10 psi, 50 psi, 100 psi, and 500 psi were measured, respectively. The main factors influencing the movable fluid saturation were analysed. The results show that the movable water saturation is strongly dependent on the centrifugal pressure used. When the centrifugal pressure increased to 100 psi, the movable fluid saturation increased sharply. The CT scanning indicates that the movable fluid saturation is mainly dominated by pore sizes of core samples, and the larger the average pore size is, the higher the fluid saturation will be.

A novel workflow for water flowback RTA analysis to rank the shale quality and estimate fracture geometry

Recently, flowback data analysis enabled us to evaluate important fracture parameters including fracture conductivity and volume in unconventional reservoirs. To perform the analysis, diagnostic plots, straight-line techniques, and history-matching techniques have been used. Immediate water and gas production usually occurs on flowback in shale gas wells. In this paper, a novel workflow is developed for the analysis of water flowback data and the early-time production of shale gas wells. This analysis then helps to define the movable water and the applicability of the soaking process on these wells.Rate transient analysis (RTA) combined with decline curve analysis (DCA) were used to analyze different shale gas wells. Effective fracture volume and geometry were calculated from the RTA analysis. The estimated ultimate water recovery (EURw) was calculated from DCA. The calculated original water-in-place (OWIP) and the EURw were compared to the injected fracturing fluid. The impact of the soaking process on the well performance was studied.Results showed that in high-quality shale wells, with no movable formation water, gas kicked off early. OWIP was equal to the total injected water volume, and boundary dominated flow (BDF) regime was observed with no transient period. The performance of these wells improved with soaking.On the other hand, in low-quality shale wells, initial formation water saturation was higher than the connate water, and water production was from both the frac fluid and formation water. Consequently, gas kicked off later and transient flow regimes were observed. Transient flow regimes were used to estimate the fracture stimulated area. In this case, however, soaking had a negative impact on the performance as the movable water saturation was high.Honoring the flowback data can be used to estimate the fracture geometry and judge the quality of the shale formation and its validity for the soaking process.

Microscopic distribution and dynamic variation of water under stress in middle and high rank coal samples

Determining the process of water migration and transformation in a pore-fracture system under stress is essential to explain the effect of water saturation on permeability and adsorption in microstructures of coals. In this study, the immovable and movable water saturation values (Sir and Sm, respectively) of middle and high rank coal samples are calculated using saturation and centrifugation methods based on nuclear magnetic resonance (NMR) technology. Micro-distribution and influencing factor of water in the initial state are analyzed by low-temperature liquid nitrogen tests. Movable and immovable saturation variations under stress are elucidated by measuring the corresponding T2 spectrum of pores with a range of diameters. The results show that the heterogeneity of irreducible water in coal samples are considerably higher than that of movable water in all the samples. The irreducible state thereafter controls the micro-distribution of water in coal samples. With the increase in coal metamorphism, the distribution heterogeneity of immovable and movable water increases and decreases, respectively. The water content variations in adsorption pore, seepage pore, and fracture all decrease linearly with the increase in Sir, indicating that the amount of immovable water determines the dynamic variation, and the distribution of initial water mainly affects the water migration process. The immovable water saturation, Sir, has distinct stress sensitivity, and the capillary bound water present in seepage pore can be transformed into movable water. The effect of stress on immovable water in larger pore, such as seepage pore, is distinctly stronger than that on immovable water in adsorption pore. This study indicates that Sir obtained by the saturation–centrifugation method is significantly higher than the actual irreducible water saturation. The bound water saturation calculated by this method is only suitable for high rank coal samples with poorly developed seepage pore.

Contributions of pore-throat size distribution to reservoir quality and fluid distribution from NMR and MIP in tight sandy conglomerate reservoirs

Overall pore-throat size distribution is a critical foundation for evaluating tight sandy conglomerate reservoirs. However, the pore-throat size cannot be easily obtained from a single technic due to the complex microstructure. In this paper, a new method was introduced to characterize the microstructure by combining thin sections, scanning electron microscopy (SEM), pressure-controlled injected mercury (PMI), rate-controlled injected mercury (RMI), and nuclear magnetic resonance (NMR). Twenty-four tight sandy conglomerate cores from the Baikouquan Formation of the Mabei oil field, northwest China, were selected to conduct the series of experiments. Overall pore-throat size distribution (TRD) was reconstructed by combining mercury injection porosimetry (MIP) with NMR with pores that were equivalent to triangular cross-section; the radii of the inscribed spheres were obtained to weaken the influence of irregular shapes by RMI. Irreducible water saturation of the cores was achieved by nitrogen displacement, which decreases with increasing of micropore proportion. An ideal relationship between permeability, movable water saturation, and micropore percentages was constructed which indicates the effect of microstructure on reservoir quality and fluid distribution in tight sandy conglomerate reservoirs.

Hydrocarbon play assessment of X-field in an Onshore Niger Delta, Nigeria

Hydrocarbon play assessment of any hydrocarbon reservoir unit depends on the porosity, permeability, hydrocarbon saturation and water saturation of petrophysical model distributions and seismic reflections of reservoir rocks. The objective of the study is to resolve the ambiguities that are associated with hydrocarbon play assessment of an X-field in the Niger Delta basin. This was achieved through the use of pre-conditioned attributes, fault delineating seismic attributes such as coherence, variance and quantitative definition of the reservoir units of petrophysical model distributions, through the adoption of an integrated methodology of 3D seismic and well log data. A quick look examination of the well log signatures revealed numerous reservoir sand units, but only three hydrocarbon-bearing reservoir sands were of interest to us (RS1, RS2 and RS3). From the quantitative interpretation of well logs, the three identified reservoir sands were evaluated in terms of porosity, permeability, hydrocarbon saturation, shale volume, movable hydrocarbon index and water saturation. Effective porosity values of 24.56, 23.01 and 24.00% were obtained for Well 1, Well 2 and Well 4, respectively. This supports the known or already established porosity range of Agbada Formation of Niger Delta with range 28–32%. The hydrocarbon saturation for RS2 is 68.51% for Well 4, for RS3 72.49% for Well 3 and for RS2 74.16% and for RS3 77.34% for Well 2. RS2 of 79.51% and RS3 of 80.99% for Well 1 were obtained. This shows how prolific the reservoir sand units are with hydrocarbon accumulation tendencies. Structural analysis revealed a highly faulted system that depicts a typical tectonic setting of the Niger Delta basin, and the computed attributes like coherence, and variance shows an optimum visualization of the faulting system. This implies that the trapping mechanism of the field is of both anticlinal and fault-assisted closure and also the viability of the reservoir units is high from the computed petrophysical parameters.

The Controls of Pore-Throat Structure on Fluid Performance in Tight Clastic Rock Reservoir: A Case from the Upper Triassic of Chang 7 Member, Ordos Basin, China

The characteristics of porosity and permeability in tight clastic rock reservoir have significant difference from those in conventional reservoir. The increased exploitation of tight gas and oil requests further understanding of fluid performance in the nanoscale pore-throat network of the tight reservoir. Typical tight sandstone and siltstone samples from Ordos Basin were investigated, and rate-controlled mercury injection capillary pressure (RMICP) and nuclear magnetic resonance (NMR) were employed in this paper, combined with helium porosity and air permeability data, to analyze the impact of pore-throat structure on the storage and seepage capacity of these tight oil reservoirs, revealing the control factors of economic petroleum production. The researches indicate that, in the tight clastic rock reservoir, largest throat is the key control on the permeability and potentially dominates the movable water saturation in the reservoir. The storage capacity of the reservoir consists of effective throat and pore space. Although it has a relatively steady and significant proportion that resulted from the throats, its variation is still dominated by the effective pores. A combination parameter (ε) that was established to be as an integrated characteristic of pore-throat structure shows effectively prediction of physical capability for hydrocarbon resource of the tight clastic rock reservoir.

Utilizing NMR Mud Logging Technology To Measure Reservoir Fundamental Parameters in Well Site

Nuclear Magnetic Resonance mud logging technology (NMR mud logging) is a new mud logging technology. Mainly applies the CPMG（Carr-Purcell-Meiboom-Gill）pulse sequence to measure transverse relaxation time (T2) of the fluid. NMR mud logging can measure drill cutting, core and sidewall core in the well site, also according to the experiment results, the sample type and size has little effect to analysis result. Through NMR logging, we can obtain several petrophysical parameters such as total porosity, effective porosity, permeability, oil saturation, water saturation, movable fluid saturation, movable oil saturation, movable water saturation, irreducible fluid saturation, irreducible oil saturation, irreducible water saturation, pore size and distribution in rock samples, etc. NMR mud logging has been used nearly 10 years in China, Sudan, Kazakhstan, etc. it plays an important role in the interpretation and evaluation of reservoir and its fluids.

Cyclic Steaming of Tar Sands Through Hydraulically Induced Fractures

Summary Fracture dimensions and process mechanisms that resulted from cyclic steam stimulation above fracturing pressure in German and Canadian tar sands are analyzed with numerical modeling. Horizontal fracture radii of 100 ft [30.5 m] and vertical fracture half-lengths of 250 ft [76.2 m] with fracture surface areas of 30,000 to 70,000 sq ft [2787 to 6503 m2] are sufficient to reproduce steam injectivity into reservoirs containing highly viscous oil and negligible amounts of movable water saturation. Mechanisms that are important during fluid loading and unloading of induced fractures include thermal expansion of tar oil, countercurrent imbibition of water and oil caused by capillary pressure effects, and fracture compressibility.

Use of Scanning Electron Microscopy to Detect Parameters for Reservoir Evaluation: ABSTRACT

Scanning electron microscopy (SEM) is a technique that permits a more detailed observation than thin-section studies for evaluating hydrocarbon reservoirs. This technique provides valuable data about textural conditions that may control log response and the reservoir's potential for damage, and it helps evaluate reactions from introduced fluids. Logs respond to many conditions that can mislead interpreters about potential well productivity. Disintegration of labile grains as well as development of pore- and grain-lining clays may produce erroneously high, movable water saturation. The breakdown of grains and pore fill create abundant microscopy, whereas pore-lining clays create high surface-area pores that effectively hold water. Formation damage can occur from mechanisms that are not related to the minerals. SEM is an important tool for identifying potential damaging conditions. Loosely attached mineral particles present a situation where they can break free and migrate into regions near a well bore. Clay-lined pores are especially sensitive to ionic changes of pore water, which may lead to clay dispersion and migration. SEM also is used to evaluate rock-fluid compatibility. Compatibility studies require before flow and after flow SEM evaluation to establish which fluids may have created damage. Several causes of damage have been documented, including new mineral growthmore » in pores, damage from crystallization of solids, and polymer residue.« less

Computer Processing of Log Data Improves Production In Chaveroo Field

Abstract Computer log analysis has proved quite successful in the Chaveroo field of Roosevelt County, N.M., in a carbonate section of low permeabilities, low porosities and variable water saturations. Production in this field occurs at about 4,300 ft in the San Andres formation, a Permian carbonate with a mixture of dolomite, anhydrite and gypsum, and lesser amounts of silica, limestone and silt. Sonic, density and neutron logs, all of which respond differently to porosity and lithology, enable a mathematical solution that defines the fractions of major lithologic components and gives the correct porosity. The Laterolog and Microlaterolog are used to solve for a movable oil relationship. The computer output is a log of bulk volume percentages of porosity, dolomite, anhydrite, gypsum and silica, and a movable oil plot. The record shows that wells completed on the basis of this computer log analysis statistically have higher initial productions and lower water cuts than wells completed on the basis of less diagnostic logging techniques. Introduction The Chaveroo pool is characterized by conditions generally considered hostile to effective log interpretation. Porosities are low-about 5 percent in most producing intervals. The intervals are predominantly dolomite, but contain unpredictably varying fractions of anhydrite, gypsum, chert, silt and limestone. Formation resistivities are high, and most wells are drilled with salt mud. However, in spite of these difficult conditions, modern logs and interpretation methods are giving information needed for efficient completion in the Chaveroo pool. The more efficient completions are obtained by avoiding water-productive intervals interspersed among the pay section of the pool. The produced water fraction is important in Chaveroo because it usually determines whether a well will flow or will require pumping. Thus, better completions are possible when water-productive intervals are distinguished from pay zones. Logs both define pay sections and identify water-productive intervals. Porosity information from logs is used with resistivity data to define fluid saturations. However, because the lithology is variable, no single log defines porosity with sufficient accuracy to reliably distinguish between pay and water. Sonic, neutron and density logs, all of which respond differently to porosity and lithology, are used in a mathematical solution that identifies major lithologic components and thus affords a reliable value of porosity. Office-based electronic computers are used to perform the mathematical solutions. The output of the computer is a log of percentages of major lithologic components and porosity, and an index of movable oil and water saturation. Statistics show that wells selectively completed on the basis of these computer logs are more efficient producers than other wells of the Chaveroo pool. These wells generally flow with higher initial oil production and produce less water. The logs and interpretation methods presented in this article are not entirely new. Earlier papers have described the primary logs, the basic interpretation methods and typical applications of computers to log studies. This study is presented to show how the various logs and methods are combined for improved well completion in the Chaveroo pool. The methods are not restricted to Chaveroo; they are similarly appropriate for other areas of complex lithology. Chaveroo Field Chaveroo field straddles the line between Chaves and Roosevelt Counties of eastern New Mexico (Fig. 1). Production is from the Slaughter zone of the San Andres formation. The main producing interval is about 650 ft below the top of the San Andres, and is at an approximate depth of 4,150 ft. The Slaughter zone, situated between two massive anhydrite sections, is 200 to 250 ft thick. It is described as a fractured, vuggy dolomite having some intercrystalline and oolitic porosity. In addition to interbedded and nodular anhydrite, the zone also contains variable but lesseramounts of gypsum, chert, silt and limestone. JPT P. 889ˆ