A case study involving power water injection in the fractured Arab-D carbonate reservoir in a Saudi Arabian field is discussed. The study was conducted to investigate the role of injection operations in the initiation and propagation of induced fractures and their communication with nearby faults, and to provide a methodology for early detection of the induced fracturing process. The study involved analysis of data gathered from step-rate, falloff, flowmeter tests, as well as injection rate and pressure data over the history of the injection operation, followed by well test modeling and hydraulic fracture modeling (HFM). Most of the eight wells studied showed the existence of fractures, corresponding to a rise in injection pressure beyond the fracturing gradient or formation parting pressure (FPP). Skin and injectivity indices obtained from the falloff tests were found to be good indicators of fracturing behavior, based on which most of the studied wells were inferred to communicate with the natural fracture system or super-permeability streaks. HFM showed that induced fractures could reach a half-length of up to 1400 ft, to various heights depending upon the injection rate and permeability. The distance to the nearest fault obtained by superimposition on a 3-D seismic interpretation was found to vary from 500 to 2,000 ft. At high injection rates, fractures were found to grow out of reservoir into underlying tight formation, which could lead to loss of injected water. For controlled fracture height, which may lead to more efficient injection operations, preparation of injection rate guidelines was recommended. Introduction Water injection is commonly practiced in depleting reservoirs for pressure support, to increase total recovery, etc. High injection pressures may sometimes exceed the formation parting pressure (FPP), creating/opening fractures that may communicate with the natural fracture system in a naturally fractured reservoir. The horizontal and vertical extent of the induced fractures, and their interaction with the natural fracture system, determines the efficiency of the injected water for the desired purpose. The present study deals with power water injection in an oil-producing, fractured carbonate reservoir in Saudi Arabia. Eight power water injection wells were identified for the study. The purpose of the study was to:investigate if the injection operations lead to the initiation and propagation of fractures,to determine the extent of the induced fractures which could establish a communication with nearby faults, and,to determine a methodology for early detection of the induced fracturing process.
Withdrawal of fluids from hydrocarbon reservoirs results in a decrease in pore pressure which in turn leads to an increase in effective stress on rock matrix. Such a situation may lead to the occurrence of pore collapse in reservoirs having weakly cemented, porous rocks. It is considered to be a potential problem in several producing reservoirs. Numerical simulation of a compacting reservoir due to pore collapse requires an appropriate constitutive model. Consequently, a constitutive model based on the concept of elasto-plasticity using isotropic hardening is developed to predict pre and post-pore collapse behavior of reservoir rocks. An experimental study is carried out on a high-porosity rock susceptible to pore collapse for different stress paths. The developed constitutive model is tested with respect to two different materials exhibiting such behavior. Parameters for the model are evaluated based on the experimental results, highlighting the procedure involved. Further, the data is used to demonstrate the strengths and the weaknesses of the constitutive model. Experimental data for the second material is obtained from literature. Satisfactory agreement is achieved between experimental data and model predictions.
TX 75083-3836, U.S.A., fax 1.972.952.9435. AbstractDrilling underbalanced is often expected to prevent formation damage, avoid lost circulation, and increase rate of penetration. However, it is also risky and may lead to borehole collapse due to lack of positive support provided by the borehole mud. Hence, its feasibility should be evaluated thoroughly through an accurate evaluation of in-situ stresses and a realistic estimation of formation rock mechanical properties. These two entities combined with the proposed borehole trajectory and mud weight design play an important role in avoiding borehole instability problems and achieving the field objectives.A study was conducted in San Joaquin field in Eastern Venezuela, to evaluate the feasibility of drilling underbalanced in highly depleted sands inter-layered with normally pressured shales. Data required for the study was obtained from five wells in the field (pressures, daily drilling reports, wireline logs, leak off tests and rock mechanical tests).A geo-stress model was developed, including pore pressure model, minimum and maximum horizontal stresses, and overburden stress. The in-situ horizontal stress directions were estimated from caliper logs, image logs and other field data. Formation rock mechanical properties were obtained using a program which utilizes the log data from the field and estimates the more representative static mechanical properties, which were then calibrated with the lab results. The combined geo-stress and mechanical properties models were calibrated with respect to drilling induced fractures and breakouts observed on the image logs. The calibrated models were used to estimate the required mud weights for drilling under various degrees of underbalance. Instabilities (breakouts) associated with each degree of underbalance were also quantified. The required mud weights to avoid shear and tensile failures, or the expected breakouts for a given mud weight, can be estimated from the generated contour plots for any borehole azimuth and inclination in the field.The results of the study were successfully implemented in the field, resulting in cost benefits of $1MM over five wells. Lost circulation was significantly avoided, and improvements in drilling rate as well as savings in rig days were achieved.The outcome of the study is expected to provide guidelines for underbalanced drilling for future wells in the area.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper presents the results and methodology of the openhole stability analysis under production scenarios of one horizontal wellbore drilled and two horizontal wellbores planned in Jurassic carbonate reservoirs of West Kuwait. The main objective of the analysis was to investigate the stability of these horizontal wellbores under multiple drawdown conditions and different reservoir depletion scenarios in order to select a suitable completion strategy which guarantees borehole stability during the productive life of the reservoirs.The stability study includes the mechanical property characterization of the formations and the in-situ stress tensor description of the oil field. The static mechanical properties were obtained from the log responses and petrophysical analysis of several wells around the field by using a micromechanical approach. This approach is based on a constitutive model describing the microscopic processes occurring in a rock sample during tri-axial loading. A postprocessing analytical program for borehole-wall failure prediction and in-situ stress estimation/calibration was used in the open-hole stability analysis. The open-hole stability analysis consisted of predicting the shear failure zone around the borehole wall under production scenarios. When the shear failure zone covers the full extent of the borehole wall, the risk of borehole collapse becomes imminent. Therefore, the maximum drawdown which maintains borehole stability was obtained for each horizontal wellbore as a function of reservoir pressure depletion.The open-hole stability analyses were also done with multiple compressive strength degradation scenarios in order to address the effect of grain-cement disintegration after acid stimulations. The results of these analyses were found to be useful in evaluating and deciding optimal well completion (open-hole, cased and cemented or slotted/expandable liners) for these wells.
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