3D Model for Rock Strength & In-Situ Stresses in the Khuff Formation of Ghawar Field, Methodologies & Applications

Abstract

Abstract Knowledge of geomechanical attributes has positive merits for drilling in hydrocarbon formations. In the Ghawar field, the Khuff is a deep, carbonate gas-bearing formation. Several development wells have been drilled into this formation and conventional logging data was acquired in them. Such data has been exploited in generating geomechanical attributes along the Khuff formation in those wells. Furthermore, hard data like minifrac tests and core mechanical properties was available in some of the wells and utilized in calibrating and validating the computed geomechanical properties. A study was conducted to develop a 3-dimensional characterization of geomechanical attributes in the Hawiyah- Khuff formation of the Ghawar field. The study was performed to provide a model that, together with existing geologic models, can be used to predict regions of drilling difficulty, provide guidance in selecting drilling bits, and help in designing drilling mud and optimizing hydraulic fracturing jobs in the field. The study established correlations between the geomechanical and the geological attributes in Hawiyah-Khuff wells. Then, these correlations were used with a pre-existing Hawiyah-Khuff geocellular model to map the geomechanical attributes away from the wells in the 3D Hawiyah-Khuff space. Rock strength, minimum and maximum horizontal stresses are among the properties that were mapped, for the first time, in the 3D Khuff horizon. The 3-D geomechanical model is consistent with observed trends in drilling difficulty, and with the requirements for hydraulic fracturing within the Khuff horizons. The model has been used in assessing the borehole stability of deviated wells planned for the field. We present some illustrations and example applications in the paper. Introduction The Ghawar giant field in Saudi Arabia is composed of six structures. The Hawiyah structure is one of them located in the southern part of the Ghawar field and it contains a gas-bearing carbonate formation, the Khuff, which is subdivided into four major depositional cycles; Khuff A, B, C and D. The basal contact of each depositional cycle in the Khuff commences with an upward transition from anhydrite to carbonate. Khuff reservoirs consist predominantly of dolomite with some limestone. The Khuff formation is considered late-Permian to early-Triassic age. Khuff-B and Khuff-C in Hawiyah structure contain hydrocarbon reservoirs of economical value. Difficulties were encountered during initial attempts of hydraulic fracturing in the Khuff. Additionally, hole stability problems were experienced during the drilling of deviated wells in the Khuff. As a result of these initial difficulties, extensive geomechanical studies were initiated to better describe and understand the rock-mechanical behavior of the Khuff reservoirs1. Since the early development stages of the Khuff reservoirs, density, resistivity and sonic logging data have been routinely acquired in the Khuff development wells. In addition, core and minifrac test data have been gathered in several development wells. Detailed geological and petrophysical reservoir characterization has been conducted, leading to a three-dimensional geocellular model of Khuff-B and Khuff-C in Hawiyah. Next, a 3D geomechanical model for Khuff-B and Khuff-C in Hawiyah has been built utilizing the data and maps available. In this paper, we will describe the work used to integrate geomechanical attributes into the existing geocellular model to arrive at a 3D geomechanical model for the Hawiyah-Khuff formation. Demonstration of how to go further in geomechanical characterization work is going to be explained too. Finally, some examples will be discussed to illustrate the applications of the above 3D geomechanical model.