Acid Fracturing of a Gas Carbonate Reservoir: The Impact of Acid Type and Lithology on Fracture Half Length and Width

Abstract

Abstract The Khuff formation is ideally made-up for acid fracturing because of the heterogeneous nature of the formation, which tends to support the created fracture conductivity. Various acid types were used to fracture this tight dolomitized formation including: 28 wt% regular HCl, emulsified acid, and in-situ gelled acid. In addition, several wells were fractured using 15 wt% HCl/9wt% formic gelled acids. However, based on production results and the large database of acid fracture treatments (more than 70 wells); it appears that there is a correlation between the acid type used and the lithology of the formation. An earlier study,1 which briefly examined this phenomenon, indicated that the emulsified acid might be more suited to low permeable zones. These low permeable rocks are predominately made up of limestone. Typically, changing the acid volumes, placement techniques, or pumping rates, has optimized acid fracture treatments by creating longer and wider fractures. However, we tend to ignore the effect of lithology in carbonate formations because we all believe that the rock interaction with the acid is not as sensitive as in sandstone formations. With this large database of acid fracture treatments, acid rock interactions were investigated to determine the relationship between lithology and acid type. This work involves correlating the designed treatment to the open-hole logs, core petro-physical data, analysis of hundreds of samples collected following acid fracture treatments, and production results. Introduction The Khuff formation is a deep gas carbonate reservoir that consists of dolomite and limestone sections underlying the giant Ghawar oil field in the eastern region of Saudi Arabia. The Ghawar field map is presented in Fig. 1, which includes the approximate boundaries of the two fields under study. The Khuff formation can have streaks of shale, anhydrite, or non-permeable intervals within the layer, which may constitute no-flow zone or fracture barriers. Figure 2 is a typical log indicating a Khuff well that is dominated with calcite, inter-dispersed with dolomite and shale stringers. This heterogeneity of the Khuff formation makes it an ideal candidate for acid fracturing even though the reservoir temperature is ranges between 280 and 300°F. Formation heterogeneity between the wells is significant with porosity intervals disappearing between offset wells.