Rates of Permeability Restoration in Tight Gas-Bearing Sandstone Cores Following Long-Duration Exposure to Cesium Formate Fluids under HPHT Conditions - A Detailed Laboratory Investigation
Authors: Downs J. D. , Howard S. K. , Anderson Z. Q. 
Published: January 2018

Deep HPHT gas/condensate wells drilled and completed in open hole with cesium formate fluids clean-up naturally over hours and sometime days after initial production start-up as the wells unload water-based fluids and filter-cake from the reservoir zone. Following natural flowing clean-up during the start-up phase the wells tend to be highly productive, with low skins, and over the long-term those fields developed entirely and solely with cesium formate fluids have a reputation for delivering the recoverable hydrocarbon reserves projected in the operators' original business plans.

Laboratory core flooding tests with cesium formate fluids attempt to simulate real well clean-ups by applying drawdown pressures across the cores to create a cleansing flow of gas or oil to bring the rock permeability back to original native levels. Such attempts are usually successful in cores flooded with clear cesium formate brines, but it is rare to hear of cores that have cleaned up 100% after long duration exposure to cesium formate drilling fluids without subsequent mild stimulation with water or dilute acid. The persistent lack of congruence between observed well clean up performance and core flooding test results with cesium formate drilling fluids suggests that the attempted laboratory simulations of natural well clean up under drawdown might be inadequate or flawed in some way. One point of concern thought worthy of further investigation has been the duration of the drawdown-induced gas or oil flows applied in laboratory core flood tests to restore permeability. Wells have the opportunity to gradually clean up over years during production while laboratory clean ups by drawdown may only be applied for minutes or hours.

The objective of the study described in this paper was to review old core flood test data to see how quickly the simulated well clean-up procedures restored original permeability in tight gas-bearing sandstone cores after exposure to high-density cesium formate fluids for at least 48 hours under HPHT conditions.

Plugs of gas-bearing low permeability (2-20 mD) sandstone containing simulated formation water at irreducible water saturation were exposed to overbalanced cesium formate fluids for 48-96 hours under HPHT reservoir conditions. The plugs were then subjected to drawdown regimes with nitrogen gas, under HPHT reservoir conditions, to simulate formation and filter-cake clean-up of an open-hole deep gas well completion at production start-up. Fluid and gas flow rates, and differential pressures across the plug, were logged whenever flow was induced through the plug, to allow estimation of the relative permeability changes in the rock throughout the test sequence.

Results were compared for HPHT core flooding tests with: 10 pore volumes of SG 2.20 cesium formate completion brine pushed through 2 mD sandstone plugs at 200° C and high pressure, followed by a 48-hour static soak period under the same conditions.10 pore volumes of SG 2.20 cesium formate completion brine pushed through 20 mD sandstone plugs at 175°C, followed by a 48-hour static soak period under the same conditions.SG 1.76 potassium/cesium formate drilling fluid circulated at 500 psi overbalance for 48 hours across the face of a 20 mD sandstone plug at 150°C, and then left static for a further 48 hours, resulting in 1.2 pore volumes of fluid loss through the core.

The straight cesium formate brines were removed quite promptly, typically within 15-30 minutes, from the 2-20 mD rock cores during drawdown. In the test with 20 mD sandstone plug and cesium formate drilling fluid the drawdown pressures were ramped up in stages from 1 psi to 100 psi during the clean-up phase but the rock plug was slower to regain its permeability. After 96 minutes of drawdown the plug had only recovered 79% of its initial relative gas permeability and clearly it was still in the process of cleaning up.

The test results provide new information about the clean-up rate of low permeability rock cores invaded by heavy cesium formate fluids under HPHT conditions and subjected to drawdown with gas.