The Effect of Cementation on Static and Dynamic Properties in Jauf and Unayzah Formations at Saudi Arabia

Abstract

Abstract The variability in mechanical properties measured on sandstones from the Jauf and Unayzah formations in Saudi Arabia is observed to be dependent on cementation. Static and dynamic measurements from triaxial testing data, measured acoustic velocities, and examination of core in thin section were used to relate the detailed microstructure and cementation to differences between static and dynamic moduli. The differences between static and dynamic moduli have numerous origins and vary from formation to formation. Properly converting from dynamic (log based) to static moduli is crucial for proper well stimulation analysis and design. Elastic moduli and velocities (compressional and shear) were measured on sixty-five plugs from five different wells encountered through Jauf and Unayzah formations. Simple models based on moduli-porosity relations failed to adequately explain our observations. We used thin section analysis to evaluate the influence of cementation on the static-dynamic moduli differences. Forty-seven samples were selected for thin section analysis and fifteen representative samples were point counted to characterize and quantify cementation. Cements in these two formations have variable composition and habits; both affect the moduli and acoustic velocities. It is not sufficient to know that cements exist; it is also necessary to know where the cement occurs. Pure quartz overgrowths cement plays a major role in increasing the moduli and velocities while clay cement plays a minor role. Simple functions suffice to describe the effect of these cements on velocity and the difference between static and dynamic moduli. Effective quartz overgrowth index was defined to quantify the effect of different type of cements. The difference between the static and dynamic moduli is reduced to a value of 2% to 15% when effective quartz overgrowth index is in the range of 16% to 26%. On the other hand, the difference increases to about 80% when high clay cements are present and the percentage of quartz overgrowth index less than 2%. Understanding these often ignored microstructural contributions, helps to understand the variation in moduli and allows for the design of more effective hydraulic fracturing treatments in the Jauf and Unayzah formations. Introduction Jauf and Unayzah reservoirs (Ghawar Field, Saudi Arabia) are deep sequences of thin bedded sandstones and shales saturated with condensate rich gas. These reservoir rocks display a wide variation in both permeability and porosity, Figure 11. Hydraulic fracturing has been utilized to enhance productivity in the Jauf sandstones of Ghawar reservoir. The complex nature of tectonic stresses, geomechanical properties, geology, coupled with varying fluid properties lead to problems of sand production and wellbore stability in the Unayzah and Jauf formations. The objective of the study is to understand the role of cementation on the variation of the static and dynamic moduli in Jauf and Unayzah formations. The difference between static and dynamic moduli is very important because the dynamic moduli obtained from well logs are used to estimate static moduli for fracture design and reservoir modeling. However, the information from acoustic well logs samples a larger volume of the reservoir than core measurements do and is less expensive2. The variation between the static and dynamic moduli can have a direct impact on hydraulic fracture design. The variation of moduli affects fracture propagation and geometry and consequently, gas production. Results of this study showed that static and dynamic elastic moduli are not only a function of porosity but also correlate with the type and amount of cementation. Quartz overgrowth and clay cements were found to be the major type of cements controlling the difference between static and dynamic moduli and velocities. These observations can assist in correlating dynamic Young's moduli derived from well logs to laboratory measured static values which are required inputs for modeling.