A Mathematical Algorithm for Modeling Geomechanical Rock Properties of the Khuff and Pre-Khuff Reservoirs in Ghawar Field

Abstract

Abstract 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.