Effective Gas-Shutoff Treatments in a Fractured Carbonate Field in Oman

Abstract

Summary A giant, fractured carbonate field in north Oman has both complex geology and complex reservoir-drive mechanisms. The upper, densely fractured layers are produced using the gas/oil gravity-drainage (GOGD) process, while the less-fractured lower set of layers is subjected to waterflooding. The production from the GOGD layers is through vertical and horizontal wells completed in a thin fracture oil rim. Gas conformance control is a challenge in many of these wells because the gas breakthrough occurs for a variety of reasons: downward movement of fracture gas/oil contact (fracture-oil-rim thinning), gas breakthrough via high-conductivity fractures (fracture gas breakthrough), zonal-isolation failure at the wellbore (mechanical gas breakthrough), and increasing gas saturation in the matrix (matrix gas breakthrough). An integrated multidisciplinary team studied well and reservoir performance and openhole (OH) and cased-hole logs to diagnose the source of higher-than-expected gas/oil ratio (GOR) in several GOGD wells. The most important logs in this work were memory-production-logging-tool (MPLT) surveys used to identify the sources of gas production and formation-microimager (FMI) logs used for fracture identification and characterization. This paper illustrates the work carried out in horizontal open-hole and vertical cased-hole completions to shut off the undesirable gas flow successfully. The horizontal wells identified with poor zonal isolation behind the liner were treated with an innovative gel gas-shutoff procedure. The merits of this procedure out-weighed those of other proposed solutions: targeted placement, a strong full-blocking gel to fill up channels behind the liner, inert particles to control fluid loss of the full-blocking gel to small fractures and the formation matrix, and displacement with an already-cured gel that could be washed out of the wellbore. Significant drops in the GORs of these wells resulted in sustained oil-production increases. This is a step change in the ability to manage detrimental gas production in this field and is expected to lead to further opportunities for improved gas management and well performance in this field and other fields where the GOGD recovery mechanism is used.