Effects of Temperature and Solution Composition on the Permeability of St. Peters Sandstone - Role of Iron (III)
Authors: Potter J.M. , Dibble W.E. , Nur A. 
Published: December 2007

Introduction
The permeability of reservoir rocks at elevated temperature is of growing importance both in oilfield development and stimulation and in geothermal energy production. Sample permeability used for reservoir evaluation often is measured at ambient conditions. However, rock and fluid properties may be altered radically as a result of enhanced rates of geochemical processes at elevated temperatures. An experimental study of quartz sandstones at reservoir conditions as a function of temperature by Aruna, who measured permeability using water, nitrogen, oil, and 2-octonol, showed a strong reversible decrease in permeability when water was used. Rock permeability to the other fluids showed essentially no change with temperature. Similar results were found by Danesh et at. and Gobran et al. Experiments by Piwinski and Netherton have shown strong decreases in permeability during continuous flow of Salton Sea brines through sandstone cores at 194 deg. F (90 deg. C). Reed has observed decreases in permeability during dissolution studies of sandpacks and cores at temperatures to 500 deg. F (260 deg. C). In this study we investigated the effects of temperature and fluid composition on the permeability of quartz sandstones, focusing on the time and flow dependence at 212 and 392 deg. F (100 and 200 deg. C).

Experimental Apparatus and Procedure
The experiments involve continuous pore fluid flow at a confining pressure of 2,940 psi (20.2 MPa) and pore pressure of 1,470 psi (10.1 MPa). A single-piston pore pressure of 1,470 psi (10.1 MPa). A single-piston metering pump with a gas-fluid separator in line to dampen pressure pulsations is used for pore fluid pumping. Fluid is preheated to within 5 deg. F (2 deg. C) pumping. Fluid is preheated to within 5 deg. F (2 deg. C) of the experimental temperature before entering the Teflon TM sleeved core holder. Metering valves control outflow rate, which is measured simultaneously using a ball float gauge. Differential pressure is measured using a differential transducer. pressure is measured using a differential transducer. The initial system was built using stainless steel tubing and preheater in the flow line system. A second system was built using titanium tubing and a special Teflon liner for the fluid preheater. Cores 1.97 in. (5 cm) long by 1 in. (2.5 cm) in diameter of clean St. Peter sandstone (greater than 99% quartz) were used. No evidence of particle migration or clay plugging has been observed in this rock. The pore fluid used was boiled, distilled, deionized water. Typically, the sleeved core was (1) placed in the pressure vessel, (2) evacuated for several hours, pressure vessel, (2) evacuated for several hours, (3) saturated at room temperature, and then the permeability was measured. Then the core was permeability was measured. Then the core was revacuated, the system heated to the desired experimental temperature, the core resaturated at temperature, and permeabilities measured continuously. Fluid viscosity and density were taken from the Handbook of Chemistry and Physics.

Results
Results using distilled water and the stainless steel experimental system are shown in Fig. 1 (Curve a). The relative permeability, normalized to a reference room temperature value, ko, decreases with time. After 400 minutes of flow, the pump was shut off. After 5 minutes more, fluid flow was resumed just long enough for an additional measurement to be made (10 to 20 seconds). This was repeated several times to determine permeability (k) during the nonflow period.
JPT
P. 905