Summary There is now an array of analytical, semianalytical, and empirical forecasting methods that can be used to history match and forecast multifractured horizontal wells (MFHWs) completed in low-permeability (tight) reservoirs. Recent developments in analytical modelling have extended model application to cases in which the fracture geometry associated with MFHWs is complex. However, analytical modelling is still primarily limited to single-phase-flow problems, which is very restrictive, and potentially inaccurate, for tight oil and liquid-rich gas reservoirs flowing at less than saturation pressure. In this work, a semianalytical method is presented for history matching and forecasting MFHWs with simple and complex fracture geometry completed in tight, black-oil reservoirs and flowing at less than the bubblepoint pressure. The linear-to-boundary (LTB) model, commonly used to model flow in the inner (stimulated) region of an MFHW, is altered to account for two-phase flow of oil and gas. The enhanced-fracture-region (EFR) case, in which both stimulated and nonstimulated regions contribute to flow, is approximated (empirically) by superposition of two modified LTB models (one representing the inner fractured region and the other the outer, nonstimulated region), and similarly altered to account for two-phase flow. An important observation is that, for MFHWs flowing at less than bubblepoint at constant flowing bottomhole pressure during transient linear flow, the slope of the square-root-of-time plot for both oil and gas phases is constant [i.e., gas/oil ratio (GOR) is constant]. The slope and intercept of the square-root-of-time plot for the primary phase (e.g., oil in the cases studied) can therefore be used to generate a forecast during the transient linear-flow period for oil and for gas (by assuming constant GOR). For boundary-dominated flow, a robust method for forecasting gas and oil was developed using material balance for both phases combined with a modified productivity-index equation that accounts for multiphase flow. A fully implicit approach has been used to solve the flow equations for oil and gas. The new modified LTB and EFR models simplify forecasting considerably for low-permeability black-oil reservoirs exhibiting multiphase flow behaviour, relative to numerical simulation, although they are not as rigorous. The new models can, however, be tied directly to the results of rate-transient analysis and are flexible enough to be applied to common conceptual models used in the literature for forecasting MFHWs under certain conditions. The new modified LTB model has been compared with both simulated and field examples. The initial results demonstrate that transient- and boundary-dominated-flow periods for oil and gas are reasonably matched with the new approach, although slight mismatches may occur, particularly during early boundary-dominated flow. The limits of the new forecasting method will continue to be explored in future work.

Summary In recent years, the tight Cardium sandstones of west central Alberta have observed significant new development with the application of multistage-fractured horizontal wells (MFHWs). This case study reviews the completion costs, economics, and production performance of 148 horizontal oil wells completed in the Pembina field halo areas (west and east), central Pembina, Willesden Green, and Garrington fields. Production analysis was performed on all wells using the Duong decline-curve-analysis technique. This technique has been used successfully on tight multistage-fractured wells in other areas, and has proved successful at predicting long-term well performance and recoveries. Wells were chosen for comparison that were in the same geological and pressure regions, had a minimum of 2 years of production information, and were stimulated with various fracture-fluid systems. In one of the analyzed areas, a definite trend was identified; it was found that wells drilled in a certain orientation performed better than wells drilled with other orientations. Completions and hydraulic-fracturing costs were gathered from public sources in order to understand the cost differences between areas and types of fracture-fluid systems. Using decline curves and cost information, full-cycle economics were run on the ’average’ well for each area in order to determine the net present value (NPV) of each area. This case study shows there is value in optimizing fracture designs through look-back studies, and there is a need to focus on more-effective fracture-treatment designs in unconventional oil development. The overall results of the study showed: (1) the choice of fracture-fluid system is a key component in optimizing economics in the Cardium halo areas; and (2) the optimum fluid system varies in different areas of the Cardium.

Performance improvement and mitigation of downhole sand plugging has resulted in minimizing the net pressure condition created within an induced-fracture network. Restricted well drawdown minimizes the physical loss of the induced stimulation, a loss which leads to downhole plugging. Using downhole chokes in liquid-unloading applications maintains high flowing sandface pressure conditions. This creates improved reservoir performance and improved well operations, and also saves costs by mitigating surface hydrate formation and fracture sand flowback.

During steam-based-oil-recovery processes (e.g., cyclic steam stimulation and steam-assisted gravity drainage), heat is transmitted from wellbore and reservoir to surrounding formations. The thermally induced deformation in shale can affect wellbore integrity and caprock integrity. In this paper, a compositional thermal strain model was proposed to quantify the thermally induced deformation in shallowly buried shale. The fabric of shale was examined and considered in the model. Thermally induced deformation in shale is contributed by the thermal deformation of shale-solid minerals, dehydration in clay, and thermal plastic strain (grain or particle rearrangement) in shale. Under drained heating, shale can contract or expand depending on its mineralogy, composition, structure, and the imposed temperature. The proposed thermal strain model was validated against the laboratory-test result of a shale sample retrieved from a heavy-oil field in the western Canada sedimentary basin.

Summary Stinger-completion technology is simple to operate and low cost, which can achieve good results in stable oil production by controlling water cut in a bottomwater reservoir. However, most previous studies of stinger completions took place in homogeneous reservoirs and are not equipped to deal with the multipoint watercresting breakthrough caused by fractures or permeability contrast in a heterogeneous bottomwater reservoir. A new improvedstinger- completion (ISC) semianalytical coupled model of reservoir and wellbore flow is established in this article, which considers the impacts of well trajectory, permeability distribution, formation damage caused by drilling and completion, and variable mass flow in a horizontal wellbore. By discrete numerical calculation of the new model, pressure, inflow, and water-cresting profiles can be acquired, and consequently, a design method of ISC is also proposed to be used successfully in a heterogeneous bottomwater reservoir for stable oil production by water-cut control and by optimizing the structure, length, and diameter of the stinger. The successful field application of ISC in the LU-X horizontal well also confirms the feasibility and effectiveness of ISC.

Summary Shale-gas production has gained worldwide attention over the past several years. Production from shale-gas reservoirs requires horizontal drilling with multiple hydraulic fracturing to obtain the most economical production. However, there are high cost and uncertainty because of many inestimable and uncertain parameters (e.g., reservoir permeability, porosity, fracture spacing, fracture half length,fracture conductivity, gas desorption, geomechanics, and existing natural fractures). Therefore, the development of a way to quantify uncertainties and optimization of shale-gas production in an efficient and practical method is clearly desirable. In this paper, we present a user-friendly and efficient framework to obtain the optimal gas-production scenario by optimizing the uncertain factors by integrating several commercial simulators, an economic model, design of experiment (DoE), and response-surface methodology (RSM) with a global optimization search engine. Specifically, we use factorial design to screen insignificant factors and find the most influential design and uncertain factors; then, we use RSM to design over those most influential factors to fit a response surface using net present value (NPV) as the objective function; finally, we identify the most economical production scenario under conditions of uncertainty. Eight uncertain parameters [i.e., porosity, permeability, reservoir thickness, reservoir pressure, bottomhole pressure (BHP), fracture spacing, fracture half-length, and fracture conductivity] with a reasonable range on the basis of Barnett-shale information are investigated. Also, different gas prices are considered for the optimization process. This framework is effective and efficient for hydraulic-fracturing- treatment design and production-scheme optimization in unconventional gas reservoirs. It can contribute to providing guidance for engineers to modify the design of a hydraulic-fracture treatment before the actual fracture treatment.

Summary This paper describes a new technique for effective placement of delineationwells on the basis of the change of the uncertainty in the key global-reservesvariable. Uncertainty is summarized through the geostatistical framework. Theauthors develop a numerical and analytical methodology that is tested onsynthetic and real petroleum case studies. The implementation isstraightforward, and the results are promising. A methodology is developed toassist in delineation-well placement. Decisions for new-well locations areassisted with a quantitative measure of the expected reduction in globaluncertainty in the volume of original oil in place (OOIP). The availablerealizations are analyzed and processed to quantify the impact of wellplacement. Variograms and other required statistics are inferred from therealizations. As a result, a gridded map of impact values is produced, fromwhich locations with the highest impact are suggested for new-well locations.Numerical and analytical approaches for the impact-map calculation are proposedand compared. Pros and cons of each approach are summarized. The numericalapproach requires a large number of realizations for effective implementationof the impact map, which might not be practically achievable. On the otherhand, the analytical approach does not require many realizations and producesstable results. In most cases, only variogram models and current well locationsare needed for the analytical impact-map computation. Although computationaltime of this approach largely depends on the model size, some options aresuggested to reduce the cost. The analytical impact calculation is developedfor the OOIP model response, in which the petroleum reservoir is defined as acomplex geological architecture with multiple structural surface constraints.Several case studies, including a real-petroleum-reservoir example, demonstratethe use of the impact map for the assessment of new delineation-well locations.The developed tool is of significant help for well placement.

Summary Constant-bottomhole-pressure (BHP) solutions to the diffusivity equation areused routinely for the analysis of well-decline performance inboundary-dominated flow (BDF). Gas-well-performance equations for constant-BHPconditions may be written in a number of ways, including rate vs.pseudopressure, rate vs. pseudotime, rate vs. material-balance pseudotimeexpressions, and, more recently, in terms of density-based rescaled exponentialmodels (Ayala and Ye 2013a, b; Ye and Ayala 2013). A significant constraint forsome of these models, especially for the rescaled exponential model, is theassumption that BHP conditions remain constant throughout the declining life ofthe well. However, the most realistic well-specification scenario is that inwhich both BHP and flow rate decline in time (i.e., avariable-pressure-drawdown/-rate condition. In this study, we explore and provethe validity of rescaled exponential and density-based gas decline models forthe analysis of variable-rate/pressure-drop gas systems in BDF. We alsodemonstrate that the constant-rate and constant-BHP solutions given by theproposed density-based decline models for gas wells under BDF areinterchangeable.

Summary A proper and effective startup of the steam-assisted-gravity-drainage (SAGD) well pairs, where both horizontal wells inject steam, is a key parameter to SAGD performance. In the past, a number of investigations have been conducted on the optimum startup strategy and operational procedures; however, fewer studies have modelled the heat requirements of the process. This study proposes an effective practical method that correlates the heat flux inside the horizontal wellbore with the temperature outside the wellbore wall. A transient conductive-heat-transfer equation in a cylindrical coordinate system is adopted for two different startup strategies: (a) variable heat flux (steam circulation in the well pair) to maintain a constant desired temperature at the wellbore wall and (b) constant heat flux (electrical heating of the well pair) to gradually increase temperature and finally reach a desired temperature on the wellbore wall. A novel and simple mathematical technique is presented for each startup approach to evaluate the instantaneous and cumulative heat requirements to keep the wellbore hot. Excellent agreements between the results of the proposed techniques and outputs of a commercial reservoir simulator are demonstrated. Accurate quantitative evaluation of heat requirements for the startup phase of SAGD enables optimum planning for steam requirements and practical design of any other heat sources employed in the process.

Summary Steam-assisted gravity drainage (SAGD) has been improved consistently by experimenting with various solutions in both oil sands and heavy-oil fields in Canada (Butler 1994; Singhal et al. 1998). The SAGD process consists of two stacked horizontal wells and is proven technology in heavy-oil fields (Butler and Yee 2000). Employing the same concept of gravity drainage, SAGD-well placement convention has been revisited and challenged many times (Birrell and Putnam 2000; Edmunds 1991; Parappilly and Zhao 2009). Known to the industry, the cross-SAGD (also called XSAGD) (Stalder 2007) and J-shaped well steam-assisted gravity drainage (JAGD) (Larter et al. 2008) processes are two examples of fit-for-purpose well design and recovery schemes that require unique drilling practices. The Celtic pool, located in Lloydminster, Saskatchewan, is an active Husky Energy Incorporated (Husky) thermal field that includes 31 SAGD well pairs. In this heavy-oil pool, one of the SAGD well pairs (I3/I4; Well I3 as producer and Well I4 as the injector) experienced production downtime soon after coming into service. Following numerous unsuccessful service jobs, several re-entry options were suggested in order to recover remaining reserves. These included sidetracking Well I3, redrilling a new well from the same pad location, or commencing a new well from a different surface location. These cases were ranked, and in 2010, on the basis of a drilling-risk assessment, the new horizontal producer well (I3A) was drilled close to the existing well pair. Well I3A was placed counter currently below the existing injector well (I4) and set successfully parallel to the abandoned Well I3. Steam conditioning was completed in the first quarter (Q) of 2011, and the new SAGD well pair (I3A/I4) has been put on production successfully. The new well pair was positioned in the opposite direction of a conventional SAGD-well-pair placement. This paper presents the original I3/I4 pair performance, the challenges, results of redrilling Well I3A, reservoir-simulation study, and recent field production of the I3A/I4 SAGD well pair.