Near-wellbore clay fines migration presents a formation damage risk in many gas wells. Fines mobilization can occur due to weakened electrostatic forces on ion exchange with an introduced fluid making them more prone to movement by viscous drag, or where the drag forces are sufficient to physically break or lift clay crystals from their original location, and distribute them throughout the pore network. Fines migration potential is typically assessed via coreflood tests on reservoir core material. Ideally, fines migration tests should be carried out on reservoir core with reservoir gas, at reservoir rate and pressure conditions, but it is often more practical (and less expensive) to scale higher field pressures down to lab scale. However, in order to reproduce the total gas flux observed in a given near-wellbore system, flow rates are routinely increased to counter the pressure decrease. This paper aims to address whether this current lab practice is valid, and to identify alternative yet practical test protocols and fluids that might more closely represent the reservoir gas properties (density and viscosity) that control the drag forces.
Reservoir core is often scarce or unavailable, which means that it can be difficult to evaluate different core flood test protocols and fluids. Outcrop samples provide a convenient alternative as they are readily available and cheap to acquire. This paper describes the first phase of a research program that aims to identify outcrop sandstones that are prone to fines migration as a result of drag forces on gas flow, and to evaluate different test protocols.
Coreflood tests were carried out on clay rich (predominantly kaolinite) Blaxter sandstone, with samples having a typical permeability of approximately 30-40mD. Potential permeability impairment from fines migration was assessed by sequential and incremental critical velocity tests at both low (290 psig) and high (1450 psig) pressure conditions, and at gas rates of up to 2 L/min. Tests were performed with nitrogen (OFN), and gaseous and supercritical carbon dioxide. In addition, hydrocarbon gas analogues (hexane and dodecane) were also evaluated as a substitute for dense gases in coreflood testing.
Initial critical rate tests using KCl brine showed the potential for salinity-related permeability damage in Blaxter sandstone cores, demonstrating that these cores are susceptible to fines migration. However, test results using anhydrous gas demonstrated that pressure and flow rate variation in the laboratory had no notable fines migration effect on the Blaxter sandstone samples. In addition, the use of different hydrocarbon gas analogues showed that even when the test fluid density is selected to so that it is similar to a liquid - supercritical CO2, or light hydrocarbons such as hexane and dodecane - fines migration is still absent even at high flow rates.
The outcrop core test results do not necessarily indicate the absence of fines migration potential in gas wells. The kaolinite fines in Blaxter sandstone may not display the well-developed clay crystal structures and morphologies normally associated with reservoir sands, and which may expose the clays to higher drag forces. The case studies presented here will aid in improving coreflood test protocols for assessing formation damage in gas wells. This improved understanding will ultimately enhance the application of core flooding as a tool for identifying formation damage in gas wells.