Steam-assisted-gravity-drainage (SAGD) processes become effective only after thermal and hydraulic communication between an injection and production well has been established during the startup operation of the well pair. Conventional steam-circulation startup operations typically take 2 to 3 months to achieve interwell communication, but reductions in the startup time can have a favorable impact on project economics. Enhancement of interwell permeability using fluid-injection (water, or steam, or solvent) strategies to promote geomechanical dilation of the oil sands has been proposed as a startup technique. These fluid-injection processes will produce complex interactions of thermal, geomechanical, and multiple-phase flow behavior in the interwell formation region. Understanding better the role that these interactions play in establishing well-pair communication will provide opportunities to improve SAGD recovery performance.
A triaxial experimental program has been designed and executed to explore whether cold-water injection would be sufficient to induce enhancements in effective permeability to water from geomechanical dilation mechanisms. Sample preparation techniques were modified to allow the preparation of reconstituted, very dense water-wet/bitumen sand specimens with different fluid saturations and almost identical porosities. Reclaimed/cleaned tailings sand from oil-sands mining operations was used to prepare artificial specimens, which are representative of McMurray Formation oil sands. A water-wet or bitumen sand core plug was then tested in an environmental chamber to simulate reservoir boundary conditions in terms of stress state, temperature, and pore pressure. A set of experiments was carried out in a triaxial cell under either initial isotropic or initial anisotropic stress state. Experimental results highlight the promising potential to dramatically enhance effective permeability to water and porosity in the dilated zone using cold-water injection at modest levels of stress anisotropy. The experimental results also provide support for the development of numerical models used in predicting SAGD startup performance and proactive utilization of the dilation as startup process for in-situ oil-sands development.
Analytical single-well models have been particularly useful in forecasting production rates and estimated ultimate recovery (EUR) for the massive number of wells in unconventional reservoirs. In this work, a physics-based decline-curve model accounting for linear flow and material balance in horizontal multistage-hydraulically-fractured wells is introduced. The main characteristics of pressure diffusion in the porous media and the fact that the reservoir is a limited resource are embedded in the functional form, such that there is a transition from transient to boundary-dominated flow and the EUR is always finite. Analogously to the frequently used Arps (1945) hyperbolic model, the new model has only three parameters, where two of them define the decline profile and the third one is a multiplier.
This model is applied to a large data set in a work flow that incorporates heuristic knowledge into the history matching and uncertainty quantification by assigning weights to rate measurements. The heuristic rules aim to lessen the effects of nonreservoir-related variations in the production data (e.g., temporary shut-in caused by fracturing in a neighboring well) and emphasize the reservoir dynamics to perform reliable predictions. However, there are additional degrees of freedom in the way these rules define the values of the weights; therefore, a criterion is established that “calibrates” the uncertainty in the probabilistic models by adjusting the parameters in the heuristic rules. Uncertainty quantification and calibration are performed using a Bayesian approach with hindcasts. This methodology is implemented in an automated framework and applied to 992 gas wells from the Barnett Shale. A comparison with the Arps (1945) hyperbolic model, the Duong (2011) model, and stretched exponential model for this data set shows that the new model is the most conservative in terms of estimated reserves.
Ojha, Shiv Prakash (University of Oklahoma) | Misra, Siddharth (University of Oklahoma) | Tinni, Ali (University of Oklahoma) | Sondergeld, Carl H (University of Oklahoma) | Rai, Chandra (University of Oklahoma)
Pore-network characteristics, such as pore-size distribution (PSD), pore connectivity, and pore complexity, along with irreducible saturations in shales, are important petrophysical parameters for accurate estimation of absolute and relative permeability curves of various phases. We apply a method for estimation of these petrophysical parameters in shales by processing the low-pressure-nitrogen-AD measurements. The method uses effective-medium theory, percolation theory, and CPA to quantify the transport properties of shales. The method has been applied to 35 samples of Eagle Ford and Wolfcamp Shales with different composition and from different maturity windows. Further, samples from the gas and oil windows of Eagle Ford Shale Formation were low-temperature plasma ashed to study the effect of the removal of organic matter on pore-network characteristics and irreducible saturations.
The estimated PSDs of condensate-window samples from Wolfcamp samples are significantly different from those of Eagle Ford samples. Our interpretation methodology indicates that the Eagle Ford samples exhibit better long-range pore connectivity and lower pore complexity compared with Wolfcamp samples. Consequently, Eagle Ford samples from oil and gas windows suggests better flow capacity compared with Wolfcamp samples from the condensate window. Moreover, the pore-network characteristics of kerogen from gas-window samples are significantly different from those of oil window samples. The estimated irreducible saturations for the samples collected from 100-ft interval in Eagle Ford gas window, 30-ft interval in Eagle Ford oil window, and the 60-ft interval in the Wolfcamp condensate window of shale formations exhibit minimal variation with depth. The samples exhibit large variations in organic content, pore connectivity, range of connected-pore network, and pore complexity that do not affect the irreducible-saturation estimates.
Yu, Wei (Texas A&M University) | Xu, Yifei (The University of Texas at Austin) | Weijermars, Ruud (Texas A&M University) | Wu, Kan (Texas A&M University) | Sepehrnoori, Kamy (The University of Texas at Austin)
The effect of well interference through fracture hits in shale reservoirs needs to be investigated because hydraulic fracturing is abundantly used in the development of unconventional oil and gas resources. Although numerous pressure tests have proved the existence of well interference, relatively few physical models exist to quantitatively simulate the pressure response of well interference. The objective of the present study is to develop a numerical compositional model in combination with a fast embedded-discrete-fracture-model (EDFM) method to simulate well interference. Through nonneighboring connections (NNCs), the fast EDFM method can easily and properly handle complex-fracture geometries, such as nonplanar hydraulic fractures and a large amount of natural fractures. Using public data for Eagle Ford tight oil, we build a reservoir model including up to three horizontal wells and five fluid pseudocomponents. The simulation results show that the connecting hydraulic fractures play a more-important role than natural fractures in declining bottomhole pressure (BHP) of the shut-in well. Matrix permeability has a relatively minor effect on pressure drawdown, and well productivity remains only slightly affected by the overall low permeability used. The BHP pressure-decline profiles change from convex to concave when the conductivity of the connecting fractures increases. At early times, the BHP of the shut-in well decreases when the number of natural fractures increases. At later times, the natural-fracture density has a lesser effect on the pressure response and no clear trend. The opening order of neighboring wells affects the well-interference intensity between the target shut-in well and the surrounding wells. After a systematic investigation of pressure drawdown in the reservoir, we formulate practical conclusions for improved production performance.
Chen, Zhiming (China University of Petroleum (Beijing)) | Liao, Xinwei (China University of Petroleum (Beijing)) | Zhao, Xiaoliang (China University of Petroleum (Beijing)) | Li, Xiaojiang (China University of Petroleum (Beijing))
Most of the work focuses on the influences of reservoir parameters on carbon-storage capacity in depleted shales, and it is very useful for selecting good candidates as repositories. Undoubtedly, the engineering parameters in shales are also very important for carbon sequestration. However, little work has discussed their impacts on carbon dioxide (CO2) storing. To improve this situation, the objective of this work is to estimate the carbon-sequestration capacity under different engineering parameters.
On the basis of a trilinear flow model, this paper studies the impacts of engineering parameters on carbon-storage potential. First, the methodology of appraising carbon-sequestration potential is introduced: (1) introducing the conceptual model, (2) developing the mathematical model, (3) obtaining the wellbore-pressure solution, (4) determining the injection time, and (5) appraising the carbon-sequestration capacity. In the conceptual model, the shale formation is divided into two subsystems and three regions: matrix subsystem, natural-fracture subsystem, hydraulic-fracture (HF) region, inner region, and outer region. With basic equations, a mathematical model is developed in these subsystems and regions. After that, on the basis of the mathematical model, CO2 storage potential in abandoned shales is investigated at different values of fracture conductivity, fracture number, fracture length, inner permeability, and wellbore length.
Although much effort has been taken to estimate the carbon-sequestration potential, little work has considered the engineering parameters. This paper innovatively estimates the carbon-sequestration capacity under different engineering parameters, which provides a guideline to selecting wells and monitoring facilities for storing CO2 in the residual-depleted shale reservoirs.
The development of unconventional shale-gas formations in North America with horizontal multifractured wells is mature enough to identify production malpractices and abnormal productivity declines generally observed within 18–24 months of initial production. The primary objective of this study is to address all known causes of these productivity declines and to develop a fully coupled geomechanical/flow simulation model to simulate these production conditions.
This model mimics the effect of depletion-induced in-situ stress variations on short-term and long-term productivity by taking into account several phenomena, such as stress-dependent matrix and natural-fracture permeability as well as reduction in hydraulic-fracture conductivity caused by proppant crushing, deformation, embedment, and fracture-face creep. Matrix-permeability evolutions, considering the conflicting effects of non-Darcy flow and compaction, have also been accounted for in this model. Numerical solutions for simplified hydraulic-fracture planar geometries are then obtained by use of a finite-element-method scheme.
A synthetic case was defined to investigate the effects of each individual phenomenon on short-term and long-term production. Results show that the combined effects of permeability alterations in matrix and natural fractures as well as conductivity losses in hydraulic fractures may result in substantial cumulative-gas-production loss. The model also reproduces familiar field-observed trends, with lower long-term production corresponding to higher drawdowns. This behavior is attributed to the stress-dependent evolution of reservoir permeability and hydraulic-fracture conductivity. The results show that ignoring the effects of any of the previous phenomena results in overestimation of ultimate recovery. Furthermore, it is shown that proper management of pressure drawdown and the penalty for lower initial production rates in unconventional shale-gas reservoirs can yield substantially higher ultimate recovery.
The model is fully versatile and allows modeling and characterization of all widely differing (on a petrophysical level) shale-gas formations as well as proppant materials used for the stimulation treatments. This integrated model can be used for optimization of key parameters during the hydraulic-fracture design, for fine tuning production history matching, and especially as a predictive tool for pressure-drawdown management.
As an unconventional rock, shale contains all the features of coalbed and tight sandstone specified as gas-adsorption capacity, microscale and nanoscale porosity, and extremely low permeability. The gas-storage mechanism of shale rocks not only is dominated by free gas in macropores and natural fractures, but also is controlled by adsorbed gas in microporous organic matter (kerogen) and clay minerals. Furthermore, Darcy’s law is no longer applicable to describe gas transport in nanopores (Javadpour 2009; Wu et al. 2015). Therefore, developing a reliable model to calculate effective porosity and permeability in nanopores considering the effects of gas adsorption, stress dependence, and non-Darcy flow is crucial to characterize properties of shale-gas reservoirs and explain gas-flow behavior in nanopores.
In this study, the simplified local density (SLD) model, which has been successfully applied to analyze gas adsorption on coal, activated carbon, and shale in recent studies, is used to analyze methane-adsorption data measured from five shale-core plugs in the laboratory (Mohammad et al. 2011; Chareonsuppanimit et al. 2012; Clarkson and Haghshenas 2016). A new approach to determine the thickness of adsorbed gas dependent on the density profile of the SLD model is proposed, which in turn provides the correction of methane adsorption to pore volume (PV). Furthermore, stress-dependence effect is incorporated into the gas-adsorption effect to generate an effective porosity function in shale rocks. In addition, non-Darcy-flow effect on gas transfer in nanopores is derived from the slit-shaped pore geometry of the SLD model and is represented by a weighted sum of second-order gas slippage and Knudsen diffusion. Consequently, the effective permeability is established as a function of the effects of gas adsorption, stress dependence, and non-Darcy flow. Moreover, the functions of effective porosity and permeability are incorporated into a numerical simulator to perform history matching for gas-production data from a horizontal well with multistage hydraulic fractures in a Barnett Shale reservoir. The simulation results properly match the gas-production data at the field scale. Finally, sensitivity studies on gas adsorption, stress dependence, and non-Darcy-flow effects are conducted to investigate their contributions to evaluating and estimating gas production from shalegas reservoirs.
The results of this study suggest that gas adsorption and non-Darcy-flow effects are two competitive aspects that have major influences on shale-gas production. The developed model including gas-adsorption, stress-dependence, and non-Darcy-flow effects provides insight into the characterization of rock properties and the description of gas-transport behavior in shale-gas reservoirs.
The upscaling of unstable immiscible flow remains an unsolved challenge for the oil industry. The absence of a reliable upscaling approach hinders effective reservoir simulation and optimization of heavy-oil recoveries by use of waterflood, polymer flood, and other chemical floods, which are inherently unstable processes. The difficulty in scaling up unstable flow lies in estimating the propagation of fingers smaller than the gridblock size. Using classical relative permeabilities obtained from stable flow analysis can lead to incorrect oil recovery and pressure drop in reservoir simulations.
Extensive experimental data in water-wet cores indicate that the heavy-oil recovery by waterfloods and polymer floods has a power-law correlation with a dimensionless number (named “viscous-finger number” in this paper), a combination of viscosity ratio, capillary number, permeability, and the cross-sectional area of the core. On the basis of the features of unstable immiscible floods, an effective-fingering model is developed in this paper. A porous-medium domain is dynamically identified as three effective regions, which are two-phase flow, oil single-phase flow, and bypassed-oil region, respectively. Flow functions are derived according to effective flows in these regions. Model parameters represent viscous-fingering strength and growth rates. The new model is capable of history matching a set of heavy-oil waterflood corefloods under different conditions. Model parameters obtained from the history match also have power-law correlations with the viscous-finger number. This model is applicable to water-wet reservoirs; it has not been tested for mixed-wet and oil-wet systems, low-interfacial-tension (IFT) environments, low permeability, and heavy-oil reservoirs with free gas cap.
In reservoir simulations, having such a correlation enables the estimation of model parameters in any gridblock of the reservoir by knowing the local viscous-finger number. The model was first applied to a heavy-oil field case with channelized permeability by waterfloods. Simulation results with the new model indicated that viscous fingering strengthened the channeling. Also, the new model shows that a lower injection rate leads to a higher oil recovery. In contrast, oil recovery in waterflooding of viscous oils is overpredicted by classical simulation methods that do not incorporate viscous fingering properly. We further showed that coarse grid simulations with the new model were able to obtain saturation and pressure maps consistent with fine-grid simulations. The new model was then used to model a real field case in the Pelican Lake heavy-oil field. It was able to match the field-production data without major adjustment of reservoir/fluid properties from the literature, showing its competence in capturing subgrid viscous-fingering effects. Overall, the new model shows encouraging capability to simulate unstable water and polymer floods in heavy-oil reservoirs, and hence can facilitate the optimization of heavy-oil enhanced-oil-recovery (EOR) projects.
Tietze, Kristina (Deutsches Geoforschungszentrum Potsdam) | Ritter, Oliver (Deutsches Geoforschungszentrum Potsdam) | Patzer, Cedric (Deutsches Geoforschungszentrum Potsdam) | Veeken, Paul C. H. (Wintershall Holding GmbH) | Verboom, Bert (Wintershall Holding GmbH)
Production from oil fields requires the monitoring of hydrocarbon saturation in the reservoir. In the Bockstedt oil field there exists a substantial difference in resistivity between an oil-filled (approximately 100–16 Ω·m) and a brine-filled (0.6 Ω·m) reservoir. Controlled-source electromagnetics (CSEM) is chosen to test whether sufficient resistivity differences can be observed by means of surface measurements. The target is a Lower Cretaceous clastic interval at an approximate depth of 1200 m. Forward modeling demonstrates that the expected resistivity changes at reservoir level cannot be resolved with a survey setup of only surface electrical sources and sensors. Therefore a borehole-to-surface technique has been developed, whereby the metal casing of an abandoned production well serves as an input electrode. CSEM surveys were acquired in 2014 and 2015 as a time-lapse baseline and monitor for both horizontal electric (Ex and Ey) components. A four-transmitter and 25-receiver configuration was deployed for these surveys. Forward modeling indicates that induction effects from metal objects, such as casings of production wells, cannot be ignored in the electromagnetics (EM) modeling. A shallow observation well was drilled in 2015 to make the collection of vertical electric field (Ez) data sets possible. A new downhole sensor was developed for this purpose. Numerical simulations suggest Ez data are more sensitive to the anticipated resistivity changes in the reservoir. Because Ez is two orders of magnitude smaller than the horizontal components, verticality is of great importance to avoid masking the Ez signal by interference from unwanted horizontal components. Similar acquisition parameters are adopted for 2014 baseline and 2015 monitor surveys to facilitate the comparisons. The repeatability is good, generating comparable response functions. A new, computationally efficient approach considers the effect of the metal casings, and is implemented in the 3D modeling and inversion codes. Preliminary resistivity models obtained from 3D CSEM inversion explain the observed data, and are in agreement with results from a 3D-seismic survey and resistivity logs in the calibration wells. Incorporation of the metal casings in the EM-modeling scheme increases the lateral continuity of inverted resistivity bodies. In November 2016, a new time-lapse acquisition campaign was undertaken to collect a second Ex/Ey monitor and the first monitor survey for the Ez component.
Liang, Baosheng (Chevron North America Exploration and Production) | Khan, Shahzad A (Chevron North America Exploration and Production) | Puspita, Sinchia Dewi (Chevron North America Exploration and Production)
It is important to determine several key parameters, such as well spacing, completions design, landing strategy, and pad sequence, for a successful full-field development of the unconventional reservoir that involves multiple wells and pads in a given area of interest. Those parameters are normally considered individually through small and simple models. In this paper, focusing on developing the whole area effectively, we provided a systematic work flow to handle such challenges together: We first recommended a top-down concept that better represents actual field development and illustrates the importance of the 3D Earth model for the unconventional reservoir; we then proposed an integrated modeling that is an iterative loop consisting of the 3D Earth model, hydraulic-fracture modeling, reservoir simulation, and uncertainty analysis.
It is uncommon to build a 3D Earth model for the unconventional reservoir mainly because of the lack of data and software capability. In this paper, we provided a cost-effective approach for the first time on the basis of a large amount of existing vertical wells, newly drilled horizontal wells, and all the data available. A 3D Earth model by use of approximately 1,100 vertical wells from the Midland Basin was presented. Such a model has a high resolution conditioned by high well density, and has an advantage of capturing hetrogeneities and interactions more than a simplified model created either from one well or low-resolution seismic interpretation. The model was fed into hydraulic-fracture modeling with the consideration of natural-fracture network and stress shadow, followed by reservoir simulation. The in-house uncertainty-analysis package that functions by experimental-design philosophy is linked to the Earth model, hydraulic-fracture modeling, and reservoir simulation. For the first time, the impacts of all the parameters together were evaluated through the final production performance. In our example, we considered completions design, discrete-fracture-network (DFN) characterization and generation, unpropped hydraulic-fracture properties, fracture compaction, and matrix permeability. The result indicated that DFN characterization is the most important parameter affecting production performance.
We applied our model and work flow to field development. Well spacing and pad sequence were studied in this paper as two examples. We demonstrated that it is important to properly consider complex interactions among multiple clusters, stages, and wells to evaluate the impacts on well spacing, completions, and development sequence.