The optimization of well spacing has become more important in unconventional shale reservoirs to efficiently design infill developments, estimate the Stimulated Reservoir Volume (SRV), and more importantly the estimation of Ultimate Oil Recovery (EUR) from each well. This paper presents a new analytical solution to estimate the start and end of pseudo-transient flow for the data production analysis where boundary-dominated flow exists in the induced fractures hence estimate the SRV for hydraulically fractured horizontal wells for unconventional shale reservoirs.
This paper presents a semi-analytical model to obtain the pressure transient response to characterize the flow and estimate the boundary effect which can be used to analyze the field data in unconventional shale reservoirs. The results from the model are compared and validated against an in-house developed numerical simulation model. The semi-analytical model is based on trilinear model where the SRV is modeled using dual-porosity idealization. The developed model involves the simulation of interference tests for two hydraulically-fractured horizontal well in unconventional shale reservoir using the real-time distributed pressure data. The proposed asymptotic solution evaluates not only the pseudo-transient in induced fractures but also the matrix.
The pressure measurements from real-time distributed pressure sensors and the production measurement using interference test provide a better understanding of the physical phenomena of the interaction between the parent and child wells in shale reservoirs. This paper presents a new model to assess the interference characteristics in horizontal wells to evaluate the optimum well spacing in unconventional shale reservoirs. It is observed that the production from a well is greatly affected by the distance of the wells, the reservoir properties between the wells, and the matrix permeability. It is presented that if the matrix permeability is lower, the start of the pseudo transient flow is sooner; therefore, the drainage volume becomes smaller. This can be observed by comparing the field data from unconventional shale reservoirs in Bakken and Eagle Ford where the matrix permeability in Bakken is higher than that of the Eagle Ford; therefore, the wells observe longer linear flow regime in higher permeability with larger SRV and in-turn larger well-spacing. The proposed asymptotic solution can also be used to analyze the field data in unconventional shale reservoirs to decipher the productivity and economics of horizontal wells.
To effectively produce from unconventional shale reservoirs, an optimum well spacing is required. This paper presents a novel asymptotic solution to characterize the flow regimes and provide a novel formulation in analyzing the pressure and rate variation with time to forecast future performance.
Ba Geri, Mohammed (Missouri University of Science and Technology) | Ellafi, Abdulaziz (University of North Dakota) | Flori, Ralph (Missouri University of Science and Technology) | Belhaij, Azmi (Saudi Metal Coating Company) | Alkamil, Ethar H. K. (University of Basrah)
Nowadays, as the worldwide consumption of hydrocarbon increases, while the conventional resources beings depleted, turning point toward unconventional reservoirs is crucial to producing more additional oil and gas from their massive reserves of hydrocarbon. As a result, exploration and operation companies gain attention recently for the investment in unconventional plays, such as shale and tight formations. A recent study by the U.S. Energy Information Administration (EIA) reported that the Middle East (ME) and North Africa (NF) region holds an enormous volume of recoverable oil and gas from unconventional resources. However, the evaluation process is at the early stage, and detailed information is still confidential with a limitation of the publication in terms of unconventional reservoirs potential. The objective of this research is to provide more information and build a comprehensive review of unconventional resources to bring the shale revolution to the ME and NF region. In addition, new opportunities, challenges, and risks will be introduced based on transferring acquiring experiences and technologies that have been applied in North American shale plays to similar formations in the ME and NF region. The workflow begins with reviewing and summarizing more than 100 conference papers, journal papers, and technical reports to gather detailed data on the geological description, reservoir characterization, geomechanical property, and operation history. Furthermore, simulation works, experimental studies, and pilot tests in the United States shale plays are used to build a database using the statistic approach to summarize and identify the range of parameters. The results are compared to similar unconventional plays in the region to establish guidelines for the exploration, development, and operation processes. This paper highlights the potential opportunities to access the unlocked formations in the region that holds substantial hydrocarbon resources.
Fluid PVT is crucial to production of a petroleum reservoir. A complete PVT study requires high quality experimental measurement combined with subsequent efforts in PVT modelling. In contrast with the relatively matured PVT study for conventional reservoirs, PVT study for shale has a number of challenges. It is difficult to get representative fluid samples; and there are various speculations on how porous media can influence fluid PVT. For modeling shale PVT, it is necessary to consider the wall effects of the rock, mainly in terms of capillary pressure and adsorption. This requires robust algorithms as well as adequate procedures to integrate available experimental information into PVT modeling. Previously, we developed equilibrium calculation algorithms with capillary pressure and adsorption and modelled adsorption equilibrium in shale. Here we further integrate them into a PVT tool for PVT simulation, analysis of shale production, and gas injection in shale. The core module in the PVT calculation is flash with capillary pressure and adsorption. A robust flash module forms the basis of PVT simulation. The capillary pressure is described through the Young-Laplace equation. For adsorption, it requires a proper workflow to bridge the limited experimental measurement and the final modeling covering a wider range of hydrocarbons. It is recommended to model the available adsorption data for light gases using a theoretical adsorption model, and then extrapolate the model parameters to heavier hydrocarbons. The generated data from the theoretical model is then fitted to the simplified and more computationally convenient Langmuir model. The flash module can also be integrated into a slimtube simulator to study the porous media effects on gas injection applications. Capillary pressure alone lowers the bubble point pressure and the extent is system dependent. Nevertheless, even for systems with a moderate decrease, the change in the PVT properties in the two-phase region cannot be overlooked. Selective adsorption alters the bulk fluid composition and lowers the heavy components concentration in general. Adsorption is generally more pronounced in the gas region whereas capillary pressure is usually more obvious in the liquid region. Regarding the influence of capillary pressure on gas injection, it can be shown that the recoveries at pressures below the minimum miscibility pressure (MMP) are changed; however, the MMP does not seem to be affected due to the vanishing of capillarity effects. For the gas injection including adsorption, the results show that the recovery decreases if adsorption is considered. This is mainly due to adsorption of heavy components, and desorption of lighter components during the process. The heavy components stay in the adsorbed phase, and will not likely be recovered even at high injection pressures. The present study integrates our previous results on algorithms and modeling into a PVT tool for analyzing shale production. It can be used to infer what the initial fluid composition is in the shale reservoir, and to analyze how capillary pressure and adsorption influence shale production during a depletion procedure. Furthermore, the tool also allows a more advanced analysis for gas injection in shale.
Seunghwan Baek and I. Yucel Akkutlu, Texas A&M University Summary A computational method using molecular-simulation data is introduced to estimate the average mean-free-path length of multicomponent hydrocarbon molecules in an organic nanochannel. Grand-canonical Monte Carlo (MC) simulation is used first to construct the equilibrium distribution of the gas molecules in the channel. These results show that the smaller the channel is, the denser the gas mixture becomes because of nanoconfinement effects. Capillary condensation occurs in the smaller channels. The fluid composition inside a channel becomes progressively heavier when the bulk-fluid pressure outside the nanopore is reduced and the lighter hydrocarbons leave the channel. The average length of the confined molecules is estimated to be an order of magnitude smaller than the theoretical value. Introduction Mean free path is a fundamental quantity that relates to various physical properties of ideal-gas molecules, such as density, viscosity (Arlemark et al. 2010; Dongari et al. 2011a, 2011b), and molecular-diffusion coefficient (Taylor and Krishna 1993). These studies have recently been extended to understand natural-gas transport in nanopores in source rocks such as shale (Civan 2010; Freeman et al. 2011). Because of their unique environment of deposition and burial, diagenesis, and hydrocarbon-fluid generation, the source rocks maintain confined pore structures in clays, solid bitumen, and kerogen. Various groups have investigated the physical and chemical properties of the hydrocarbon fluids and water under the effects of nanoconfinement (Javadpour et al. 2007; Freeman et al. 2011; Kang et al. 2011; Ambrose et al. 2012). Kerogen, an insoluble organic constituent of the source rock, has in particular received much interest because of its nanopores with a large specific surface area contributing to the storage of the hydrocarbon fluids. Kerogen holds significant volumes of hydrocarbons confined in its organic amorphous nanostructure at high pressure and temperature (Kang et al. 2011; Ambrose et al. 2012; Baek and Akkutlu 2019a, 2019b). However, study of the behavior of complex multicomponent hydrocarbons has rarely been conducted, although an understanding of the compositional effects on the transport of hydrocarbons is a prerequisite for resource assessment and efficient production (Clarkson and Bustin 2000; Kang et al. 2011; Papaioannou and Kausik 2016; Baek and Akkutlu 2019a).
Hydraulic fracturing is the most prominent technique for increasing well productivity in shale oil and gas reservoirs. Spacing between perforation clusters, with Plug-and-Perf (PnP) fracturing method also believed to be spacing of fractures, is one of the parameters that need to be optimized in fracturing design. This work presents an analytical method to optimize fracture spacing based on the assessment of production data from multi-fractured horizontal wells. Five hydraulically fractured horizontal wells completed in a high clay-content shale formation were considered as examples in this study. Based on the theory of pseudo-steady state (PSS) flow, oil well productivity was investigated with special focus on perforation cluster spacing. Wells exhibiting linear flow in rate transient analyses are believed to have a potential of productivity improvement with shorter fracture spacings. Oil productivity potential with closer fracture spacing was identified for wells in the same reservoir. Result of analyses shows that some wells experienced linear transient flow, indicating significant separation between hydraulic fractures. Use of an analytical well productivity model to simulate pseudo-steady production indicates that fracture spacing could have been reduced to improve well productivity. These wells can be considered as candidates for re-fracturing if possible. Future wells to be drilled in the area nearby should be completed with shorter spacing of perforation clusters. Some wells in the case fields do not show linear transient flow, indicating interferences between either hydraulic fractures and/or natural fractures. These wells should not be refractured. New wells near these wells should not be completed with fracture spacing less than the spacing values used in the existing wells.
A major outstanding challenge in developing unconventional wells is determining the optimal cluster spacing. The spacing between perforation clusters influences hydraulic fracture geometry, drainage volume, production rates, and the estimated ultimate recovery (EUR) of a well. This paper systematically examines the impact of cluster spacing in the Eagle Ford shale wells by calibrating fracture geometry and fracture/reservoir properties using field injection and production data and evaluating the optimal cluster spacing under different reservoir conditions.
We explore a sequential technique to evaluate and optimize cluster spacing using a controlled field test at the Eagle Ford field. This study first identifies the fracture geometry by history matching the field injection treatment pressure. Using the rapid Fast Marching Method based flow simulation and Pareto-based multi-objective history matching, we match the well drainage volume and the cumulative production to calibrate the fracture and SRV properties. The impact of cluster spacing on the EUR are examined using the calibrated models. We run injection and production forecasts for various cluster spacing to investigate optimal completion under different reservoir conditions.
The unique set of injection and production data used for this study includes two horizontal wells completed side by side. The well with tighter cluster spacing has larger drainage volume and better production performance. This is because of the increased fracture complexity in spite of the impact of stress shadow effects leading to shorter fractures. The calibrated models suggest that most of the fractures are planar in the Eagle Ford shale. The well with wider cluster spacing tends to develop longer fractures but the well with tighter cluster spacing has better stimulated reservoir volume with enhanced permeability, thus resulting in better drainage volume and production performance. From the optimization runs under different reservoir conditions, our results seem to indicate that when natural fractures are present or when stress anisotropy is high with no natural fractures, the wells with tighter cluster spacing tend to outperform the wells with wider cluster spacing. However, severe stress shadow effect is observed when stress anisotropy is low with no natural fractures, likely making tighter cluster spacing wells less favorable.
The calibrated fracture geometries and properties with a unique set of Eagle Ford field data explain the performance variation for completions using different cluster spacing within the reservoir and provides insight into optimal cluster spacing under different reservoir conditions (low vs high stress anisotropy and with/without natural fractures).
Production data and analytical models derived from coupling the linear flow in the reservoir and the linear flow in hydraulic fractures were used in this study to optimize fracture spacing for maximizing productivity of shale oil and gas wells through refracturing. This study concludes that productivity of multi-fractured horizontal wells is inversely proportional to the fracture spacing. The shortest possible fracture spacing should be used to maximize well productivity through refracturing. This supports the practice of massive volume fracturing where as many as perforation clusters with the shortest possible spacing are used for pumping massive proppant into the created hydraulic fractures. Production data analysis indicates that the multi-fractured horizontal oil and gas wells could have higher productivity if they were fractured with less perforation cluster spacing. Mathematical model analysis implies that reducing the cluster spacing from 70 f t t o 15 f t t h r o u g h r e f r a c t u r i n g c a n d o u b l e d w e l l p r o d u c t i v i t y, w i t h t h e M i n i m u m Re q u i r e d C l u s t e r S p a c i n g (MRCS) determined by well completion constraints (packers, perforation clusters, and casing couplings). Result can be checked for fracture trend interference on the basis of analyses of pressure transient data or production data.
Molecular diffusion plays an important role in oil and gas migration and transport in tight shale formations. However, there are insufficient reference data in the literature to specify the diffusion coefficients within a porous media. This study aims at calculating diffusion coefficients of shale gas, shale condensate, and shale oil at reservoir conditions with CO2 injection for EOR/EGR. The large nano-confinement effects including large gas-oil capillary pressure and critical property shifts on diffusion coefficient are examined. An effective diffusion coefficient that describes the diffusion behavior in a tight porous solid is estimated by using tortuosity-porosity relations as well as the measured shale tortuosity from 3D imaging techniques. The results indicated that nano-confinement could affect the diffusion behavior through altering the phase properties, such as phase compositions and densities. Compared to bulk phase diffusivity, the effective diffusion coefficient in a porous shale rock is reduce by 102 to 104 times as porosity decreases from 0.1 to 0.03.
Application of horizontal wells and multi-stage fracturing has enabled oil recovery from extremely low permeability shale oil reservoirs, but the decline in production rate is more than two thirds in the first two years. We are trying to develop chemicals that can be injected into old wells to stimulate oil production before putting the well back in production. The goal of this work is to evaluate chemical blends for such a process at the laboratory scale. The chemical blend contains surfactants, a weak acid, a potential determining ion, and a solvent. Six different solvents were screened: Cyclohexane, D-Limonene, Dodecane, Kerosene, Turpentine, and Green Solvent®. Most of the chemical blends with the solvents extracted about 60% of the oil from shale chips, but the Green Solvent® extracted about 84%. Spontaneous imbibition tests were performed with outcrop Mancos shale cores. Oil was injected into these outcrop cores at a high pressure. NMR T2 distributions were measured for the cores in the original dry state, after oil injection and after imbibition. The aqueous phase from the chemical blend imbibed into the cores and pushed out a part of the oil and gas present in the cores. The surfactant in these blends can change wettability and interfacial tension. The solvent can mix with the oil and solubilize organic solid residues such as asphaltenes. The weak acid can dissolve a part of the carbonate minerals and improve permeability. The synergy can make these chemical blends strong candidates to stimulate oil recovery in shale formations.
Oil production from shale and tight formations will increase to more than 6 million barrels per day (b/d) in the coming decade, making up most of total U.S. oil production (> 50%). However, achieving an accurate formation evaluation of shale faces many complex challenges. One of the complexities is the accurate estimation of shale properties from well logs, which is initially designed for conventional reservoirs. When we use the well logs to obtain shale properties, they often cause some deviations. Therefore, in this work, we combine cores and well logs together to provide a more accurate guideline for estimation of total organic carbon, which is primarily of interest to petroleum geochemists and geologists.
Our work is based on Archie's equation. Resistivity log will lead to some incorrect results, such as total resistivity, when we follow the conventional interpretation procedure in well logs. Porosity is another complex parameter, which cannot be determined only by well log, i.e. density, NMR, and Neutron log. Therefore, the flowchart of TOC calculation includes five main parts: (I) the shale content calculation using Gamma log; (II) the determination of shale distributions using Density and Neutron logs and cross-plot; (III) the calculation of total resistivity at different distribution types; (IV) obtaining porosity using core analysis, NMR and density logs; and (V) the calculation of TOC from modified Archie's equation.
The results indicate that the shale content has a strong effect on estimation of water saturation and hydrocarbon saturation. Especially, the effect of shale content is exacerbated at a low water saturation. A more accurate flowchart for TOC calculation is established. Based on Archie's equation, we modify total resistivity and porosity by combining Gamma Log, Density Log, Neutron Log, NMR Log, and Cross-plot. An easier way to estimate porosity is provided. We combine the matrix density and kerogen density together and obtain them from core analysis. Poupon's et al. (1954) laminar model has some limitations when applying in shale reservoirs, especially at a low porosity.
Literature surveys show few studies on the flowchart of TOC calculation in shale reservoirs. This paper provides some insights into challenges of well logs, core analysis in shale reservoirs and a more accurate guideline of TOC calculation in shale reservoirs.