TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe Hawiyah Jauf Reservoir is a complex structuralstratigraphic trap located along the east flank of the uplifted Ghawar structure. The two important aspects of this reservoir are fieldwide composition variation and reservoir compartmentalization. The reservoir fluid samples obtained during developmental drilling show remarkable differences in fluid properties and condensate yields. The condensate yield of collected samples varies form more than 250 STB/MMscf to less than 100 STB/MMscf, and the reservoir fluid at the gas-water contact exhibits the near-critical behavior. Furthermore, the complicated fault system subdivides reservoir into several major compartments. In order to model the fieldwide composition variation, all fluid samples must be characterized reasonably well by using one equation of state (EOS) fluid model. An eight-component EOS fluid model has been generated based on four fluid samples. A geological model was built to honor the faults that were interpreted from the recent 3D seismic survey.A series of prediction runs are planned to forecast the reservoir behavior and hydrocarbon recoveries under various operational scenarios including simple depletion as well as cycling at different stages of production. Sensitivity runs are also made to investigate the impact of uncertainties associated with geologic and reservoir properties.
As part of gas production enhancement and quick recovery of reserves initiative, hydraulic fracturing campaign was initiated in 1998 to treat the sandstone and carbonate formations in the Ghawar gas fields in Saudi Arabia. To date, numerous wells have been matrix acidized, acid fractured, and proppant fractured. Many different procedures and fracturing techniques have been adopted as part of learning process to optimize candidate selection, perforation strategy, fracture design, and implementation.1,2 The incremental production from the Khuff carbonate gas ascertains the success of the acid fracturing treatments.3 For the Pre-Khuff sandstone reservoirs, because of the unconsolidated nature of some high potential sandstone layers, indirect fracturing technique has been adopted where the consolidated, low porosity section is perforated and from where the induced fracture is initiated.4 This procedure curtails the risk of sand production. Very recently, fracturing wells with frac&pack technique and completing them with mechanical screen are being implemented. Successes on these Pre-Khuff treatments are usually judged by sand free production rate and conditions and by comparing post-fracture production increase with simulated, pre-fracture production based on reservoir properties calibrated by post-fracture production match. In most cases this gain is significant to justify the expenses of the fracture treatment. However, the priority in Saudi Aramco is to recover the required volume of gas to feed its giant gas plants.This paper presents field cases from the carbonate and sandstone reservoirs treated with acid and proppant, respectively and provides a general picture of how all the fracture treatments are currently being conducted. Introduction Gas wells in the Ghawar field are currently fractured to enhance gas recovery and feed the present and upcoming giant gas plants. To date over 50 wells have been acid fractured in the Khuff carbonates and about 15 wells have been propped fractured in the Pre-Khuff sandstones.5 The Jauf and 'Unayzah reservoirs form the productive Pre-Khuff formation where high permeability, unconsolidated sand layers are often encountered. One major objective of fracturing sandstone is also to reduce sand production to the minimal acceptable level. This paper presents case histories from the fracture treatments. Examples are selected such that the variation of reservoir properties and different fracture techniques are represented. Due to the vast areal spread of the reservoirs and the thickness of the formations, the lateral and vertical variation of important reservoir flow and mechanical properties such as porosity, permeability, in-situ stress conditions, Young's modulus, fluid characterization, etc., change. It is therefore a challenge to optimize the treatments and come up with unique way for completion and production enhancement. The examples depicted cover the property variations and provide a comprehensive understanding on the treatment approach. Examples provided presents geological description, geomechanical property evaluation, perforation selection strategy, treatment design and execution consideration, and some post fracture evaluation. Both successes and marginal successes in fracturing treatments are presented and the reasons for either are discussed. Three-dimensional hydraulic fracturing model to simulate acid reaction and proppant transport and reservoir simulation models for production and transient data evaluation have been used for the analysis. Finally, conclusions are drawn to provide guidelines to best practices for enhanced gas recovery.
The Khuff reservoir in South Ghawar, Saudi Arabia is a heterogeneous tight carbonate reservoir. Acid fracturing program has been initiated to enhance gas production from this reservoir to deliver 1.2 BSCFD of gas. The complex nature of the tectonic stresses, geomechanical properties, reservoir geology and quality, coupled with aerially varying fluid properties (H2S) have sometimes reached the limits of reservoir simulation and stimulation. During the early stages of development, numerous diagnostic tests were conducted on every well before, during, and after fracture treatment. These tests include pre- and post-fracture pressure transient analysis, radioactive tracer or temperature log runs during mini and main fracture treatments, production tests, and production logging. Several acid fracturing fluids, such as, 28% HCl, emulsified 28% and 15% HCl, were applied to achieve optimum fracture length and conductivity, and avoid CO2 corrosion. Complete reservoir characterization, history matching of actual fracture treatment and production forecasts were performed for every well. The objective of these tests and detailed evaluation is to optimize future acid fracture design. This paper presents in detail the integrated optimization process of acid fracturing in the Khuff reservoir. The paper demonstrates that early investment in technology, management of uncertainties, and state of the art reservoir and fracture modeling significantly improved the optimization process. The paper presents detailed assessment of several key wells, comparison between various acid fracturing fluids and techniques, and calibration of results of the stress and fracture models. The paper also discusses the need and importance of some of the tests and the quality of the results that may be obtained from them. Finally, the paper presents reservoir modeling and history matching of acid fracture treatment using state of the art coupled geomechanical single well simulation model. Predictions of acid fracture performance versus time under the Khuff abnormal stress conditions are presented. Introduction Since the beginning of the fracturing campaign in 1999 in order to enhance gas production and deliver the required gas volume to the gas plant, over 20 wells have been acid fractured in the South Ghawar carbonate reservoir.1 The carbonate reservoir is tectonically active and high stress is encountered in various part of the formation. Thus, at the beginning of the fracturing program, study and research initiatives were undertaken in order to better understand the geomechanical properties and reservoir characteristics.2 Numerous microfrac and minifrac tests were conducted to develop and calibrate the geomechanical model3 used to predict reservoir in-situ properties. This current model thus predicts the main mechanical properties such as in-situ stress, Young's modulus, and Poisson's ratio with precision, which are used by the fracturing model to compute fracture initiation and propagation. At the initial stages of the project, large volumes of pad and acid sequences were used to fracture the reservoir. 28% strength HCl was commonly used with volumes ranging between 1,500 and 1,800 gals per feet of net interval treated. Along with numerous sensitivities performed through stimulation designs, different methods to stimulate the wells were implemented in the field. The objective was to optimize the treatments in terms of cost, effectiveness of stimulation, and meeting the needs of future completion requirements. Because of the heterogeneity of the reservoir and the risk of developing condensate bank near well due to pressure dropping below dew point, the optimization process has been a challenge to the reservoir and stimulation engineers. Nevertheless, the improvement in applying and optimizing stimulation technology has been distinct and the work for continuous research and development will be on going, particularly when these wells are put into production.
The Khuff and Pre-Khuff are deep gas condensate reservoirs under active tectonic stress environment. The reservoirs are under development using horizontal wells and vertical wells with hydraulic fracturing. Modeling geomechanical rock properties accurately is essential for ensuring a successful frac job design and execution. During the last two years, a large amount of additional lab and field information has become available. Integration of all the data was conducted for better estimation of in-situ geomechanical rock properties. This paper presents the results of a mathematical algorithm for calculating the geomechanical rock properties for the Khuff and Pre-Khuff reservoirs in the Ghawar field. The model is derived from the classical poroelastic model in addition to a tectonic strain component as proposed by Prats and Warpinski. The model was calibrated to lab data as well as to the results of several Microfrac and Minifrac field tests. The model was further improved by calibrating it with actual history-matched frac data. The algorithm describes a methodology for systematically calculating geomechanical rock properties and in-situ minimum horizontal stress magnitude from sonic shear and compression log data. The paper also describes a detailed history-matching algorithm for Minifrac and frac data using a 3-D frac simulator. The results show that the minimum in-situ stress in the Khuff and Pre-Khuff reservoirs is governed by the tectonic effect, which is Young's modulus dependent. Detailed analysis and well examples are presented. Introduction Well stimulation technology has proven to be successful in improving hydrocarbon recovery.1 Many wells are stimulated to increase productivity and recovery. Two types of well stimulation techniques are generally adopted, viz., hydraulic fracturing and acid fracturing. The first type is used in sandstone reservoirs and high-conductivity proppants are used to keep open the fracture initiated and propagated mainly by the pad fluid pumped prior to proppants. The second type is used for carbonate reservoirs where acid is used to react with the rock once a fracture is created by the viscous pad. The reaction of the acid etches the fracture walls and matrix rock creating a conductive path from the reservoir to the wellbore. Saudi ARAMCO has initiated an acid fracturing program to treat the Khuff carbonates and Pre-Khuff sandstone reservoirs in the Ghawar field in the eastern province of Saudi Arabia. The fracture treatments conducted thus far have resulted in very encouraging gas rate and well productivity. In this paper, we discuss some of the main reservoir properties that impact fracture and production behavior, which are the geomechanical properties. We will provide a review of the mathematical models used to generate the data. We will also provide a systematic approach for calibrating and improving the model by integrating and history matching field data. Actual field examples will be provided to illustrate the process. Both reservoir properties, particularly the mechanical properties, and perforation placements dictate the geometry of the fracture and its effectiveness. Placement of perforations is controllable and should be based on accurate prediction of reservoir flow and geomechanical properties. Therefore, it becomes very important to accurately predict geomechanical properties. Reservoir Geology The focus of this paper is on the Khuff and Pre-Khuff Jauf reservoirs in Ghawar field. The structure map is presented in Fig. 1.
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