Using Microgels to Shut Off Water in a Gas Storage Well

Abstract

Abstract The present paper describes the first field application of a new water shutoff technology using size-controlled microgels for the treatment of a gas storage well. The treatment was designed from an integrated study combining laboratory coreflood experiments and near-wellbore reservoir simulations. The candidate well (open hole completion with liner) was drilled in a sandstone reservoir formation made of a succession of layers with different petrophysical quality all connected each other. The presence of a thin high-permeability streak located in the bottom part of the open interval was assumed to be the main factor of excessive water production. A bullhead option was chosen for the treatment. Microgel size (around 2 micron) favored their placement in the high-permeability layer, whereas the penetration in the rest of the reservoir was expected to be very superficial. The treatment was performed in June 2005. Due to pressure constraints, the volume of the treatment had to be reduced to 26 m3 only. A backflush of gas was necessary to recover gas injectivity. The well was kept on production during the whole winter season 2005–2006. Water production was significantly reduced thus enabling higher gas rate production. A positive impact on sand production was also observed. Well behavior was in good agreement with model predictions. Introduction Water intrusion from aquifer in underground gas storage wells affects well performances more especially when the Gas-Water Contact is high in the reservoir and the gas rates due to peaks of demand are intense. Some pioneer field operations have shown that polymer technology can be an efficient way to solve this problem.1,2 The principle of the treatment is shown in Figs. 1 and 2. The polymer injected into the open interval invades the different layers surrounding the wellbore with a deeper invasion of the higher permeability layers, which are frequently the main water producing zones. This type of product adsorbs on the formation rock almost irreversibly and induces a selective reduction of the relative permeability to water with respect to the relative permeability to oil or to gas. This unique property of water soluble polymers and gels is well documented in the petroleum literature, as "RPM" (Relative Permeability Modification) or "DPR" (Disproportionate Permeability Reduction). Indeed, although the origin of this phenomenon remains controversial, there is a large consensus about the reality of this property, which has been observed during either water/oil or water/gas two-phase-flow coreflood experiments with many different core materials and polymer/gel species.3–12 The combination of a polymer/gel favorable placement in the water-producing layers together with RPM effect induces lower water cut production after treatment. If the loss of Productivity Index can be compensated by higher drawdown, the treated well can produce more oil or more gas. Liang, Lee and Seright pointed out that RPM effect is not enough to guarantee a successful treatment, especially in radial flow conditions.13 In fact , a perfect RPM product, which reduces the relative permeability to water without inducing any change in the curves of relative permeability to oil or to gas, may affect the effective permeability to oil or to gas through an increase in water saturation. This occurs when the fractional flow of water from the oil producing layers is significant. The water blocking effect increases with the depth of penetration of the polymer/gel in the layer and with the intensity of the permeability reduction to water. Therefore, preventing a deep penetration of RPM products into the oil/gas layers can be considered as mandatory to achieve a good water shutoff treatment.