Effective Gas Shutoff Treatments in a Fractured Carbonate Field in Oman

Abstract

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 water flooding. 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 as 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) andincreasing gas saturation in the matrix (matrix gas breakthrough). An integrated multidisciplinary team studied well and reservoir performance and open-hole and cased-hole logs to diagnose the source of higher than expected GOR in several GOGD wells. The most important logs in this work were MPLT surveys used to identify the sources of gas production and FMI logs used for fracture identification and characterization. This paper illustrates the work carried out in horizontal openhole and vertical cased-hole completions in a giant fractured carbonate field to successfully shut-off the undesirable gas flow. The horizontal wells identified with poor zonal isolation behind the liner were treated with an innovative gel gas shut-off procedure. The merits of this procedure outweighed other 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 which could be washed out of the wellbore. Significant drops in the GORs of these wells resulted in sustained oil production gains. 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. Introduction The giant fractured carbonate field was discovered in 1964 and came on stream three years later. The field has 7 reservoir layers A - G and multiple subunits within each layer. The upper layers A, C, D and E1/E2 are more intensely fractured than lower layers E3/E4, F and G reservoirs. Initial production from the reservoirs (1967–1970) was by natural depletion, supported by gas injection in the A reservoir unit from 1968 onwards. After this initial period of gas injection, water injection was implemented in the A, C, D and E reservoirs (1970–1984). Previously unknown fracture networks in these layers resulted in rapid water breakthrough. This was followed by GOGD (Gas-Oil-Gravity-Drainage) development (1983–1998), which was successful in arresting the decline in the oil production. Following a simulation study in 1996, it was decided to implement a line-drive waterflood with horizontal wells in those layers considered to be sparsely fractured. Because GOGD is not effective in sparsely fractured reservoirs, waterflooding these layers was expected to substantially increase recovery in those layers. Since 1997, field development and operation has utilized this combination of GOGD and localised waterflood. A recent review of production and petrophysical data showed that the fracture spacing varies significantly both hortizontally and vertically. This impacts the GOGD efficiency and recovery factor, since GOGD is inefficient where fracture spacing is larger, as the oil has to travel long distances through the matrix to the nearest fracture set in order to be recovered.

Inflow Profile Control in Horizontal Wells in a Fractured Carbonate Using Swellable Elastomers

“…Where excessive gas is produced due to gas breakthrough in fractured reservoir layers, or via a poorly cemented well bore, chemical gas shut off techniques have been used [3].…”