Chemical Stimulation of Gas/Condensate Reservoirs

Abstract

Abstract Significant productivity loss occurs in gas-condensate wells when the bottom hole flowing pressure drops below the dewpoint pressure. The decline in productivity is due to near-well accumulation of condensate in the reservoir rock, which is significant even for wells producing very lean gas with liquid dropout values less than 1%. Many different methods such as hydraulic fracturing, dry gas injection and solvent injection have been proposed and implemented to stimulate such wells. However, all of these methods offer short-lived stimulation and are sometimes not profitable. New experimental core flooding data using chemical treatments show that the steady-state gas and condensate relative permeability in both outcrop and reservoir sandstones can be increased by a factor of 2 to 3 over a wide range of temperature (145 to 275 °F). Spectroscopic data show that the sandstone surface remains modified by the chemical even after flooding the core with large volumes of gas. A relative permeability model that includes effects such as the decrease in the residual condensate saturation after treatment and the effect of capillary number is presented. Fine-grid compositional simulations of a single-well treatment were done using the calibrated relative permeability model to investigate the performance of chemical treatments under field conditions as a function of variables such as treatment radius. These simulations show that chemical treatments have the potential to greatly increase production at low cost relative to the increased revenue since only the near-well region blocked by the condensate needs to be treated. Introduction Gas-condensate reservoirs show a decline in productivity when the bottom hole flowing pressure drops below the dewpoint pressure of the fluid. Liquid condensate builds up near the well restricting the flow of gas. In other words, capillary forces trap some of the condensate phase in the pores, which causes a reduction in the relative permeability of both the gas and condensate. A well-established example of condensate blocking and a reduction in productivity of more than a factor of two occurred in the Arun field.1 The degree of condensate blocking depends on a combination of factors including fluid properties, formation characteristics, flow rate and pressure.2–6 Since the reduction in well productivity is primarily associated with the reduction in gas relative permeability, a great deal of effort has gone into measuring and modeling the relative permeability of gas-condensate fluids. Several recent studies have been made using the pseudo pressure or flash method for measuring steady state gas and condensate relative permeability.2–13 In this method, a gas mixture above its dew point pressure is flashed into the core at a reservoir pressure less than the dew point pressure and flowed until steady state is reached. Several studies have been done to quantify the relative permeability of gas and condensate. Initially, the studies were done at low pressure and temperature.14 Later studies were done at reservoir conditionswith synthetic fluids4,6,7 as well as with reservoir fluids.15 Various parameters such as interfacial tension,16 high flow rates,17,18 non-Darcy effects,19 fluid composition16,20 and rock type5,20,21 have been investigated. Several strategies have been tried and tested for stimulating gas-condensate wells with limited success.22 Gas cycling23,24 allows the pressure to be maintained above the dew point but may not be economical, especially late in the life of the reservoir when large quantities of injected gas are required to maintain the pressure above dew point. Hydraulic fracturing25,26 and horizontal wells27 both increase the flow area and thus production rates. However, fracturing has limitations such as in wells with bottom water. Condensate still builds up around fractured wells or horizontal wells although it takes a longer time for the bank to form. The benefit must be compared to increased cost. Solvents such as methanol have been tested in both the laboratory2,3,28 and field29 and found to restore gas productivity for a short period of time.