Peace River Tar Flow Experiments Under in Situ Conditions

Abstract

Introduction In a preceding paper we described some chemical properties of Peace River tars, i.e., solvation properties of asphaltenes and their reflection of the thermal history of the tar in the reservoir. It was suggested that the thermal alteration observed in tar samples produced during the Shell 1973–74 field test at Peace River could lead to important changes in the flow properties of the tar relative to its initial unaltered state. With the above factors in mind, we have performed a series of flow experiments with Peace performed a series of flow experiments with Peace River cores preserved as nearly as possible in their natural state. The conditions chosen were such as to simulate the hot zone present in the Bullhead Sand reservoir during the 1973–74 field test. FLOW EXPERIMENTS Experimental Conditions All flow runs were made at temperatures of 380 - 389 deg. F (193 - 198 deg. C). The sand plugs were subjected to an isostatic effective stress of appx. 1,100 psig (7,584 kPa), with the overburden pressure held psig (7,584 kPa), with the overburden pressure held at appx. 1,500 psig (10,342 kPa) and the back pressure on the fluids at appx. 400 psig (2,758 kPa). This was intended to simulate original in-situ burial conditions. Plug handling and mounting procedures were developed to maintain the lightly consolidated Peace River core materials with their original Peace River core materials with their original interstitial fluids in a relatively undisturbed condition. Details are given in Appendix A. An initial objective was to obtain mineral-free tar from Peace River core material, employing separation procedures which would minimize core positional changes of the native tar. Experimental positional changes of the native tar. Experimental extraction procedures and resulting tar properties are described in Appendix B. As the experimental flow studies were expanded, other forms of Peace River tar, for example, deasphalted tar, cold- and thermally-produced tar from the Peace River pilot tests, were also used. The laboratory brine used in all flow measurements on Peace River cores was fixed at a concentration of 25,000 ppm sodium chloride. This compares with field brine concentrations varying from 27,488 to 35,216 ppm total dissolved solids as reported from DST analyses. A 50,000 ppm brine solution was used in later experiments involving Berea cores. The brines were deaerated before use in the flow system. Two approaches were employed in the flow studies:two phase (tar/brine) flow in single pass-through, steady state permeability determinations pass-through, steady state permeability determinations andone phase (tar) continuous recycle flow. Fixed flow rates for brine and/or tar were established by means of precalibrated positive displacement pumps. Resulting pressure drops cross the cores were determined by means of a multiple range pressure transducer unit. special high pressure fillings provided electrical insulation between the pressure cells and the outflow fluid lines from the plugs. Platinum screens positioned at the plug faces, coupled with these positioned at the plug faces, coupled with these fittings, provided capability of measuring plug resistances and determinations of fluid saturations during fluid flow. Archie saturation exponents were obtained independently and varied from 1.81 to 1.91 depending on the particular plug. In some cases, fluid saturations were also determined at the end flow runs by standard extraction methods. A description of the permeability cells and their various elements is provided in Appendix C. Flow Results A tabulation of the runs, including data on cores and tar used, is presented in Table 1. A more detailed description of some of the flow tests follows.

Steady-State Tar/Water Relative Permeabilities in Peace River Cores at Elevated Temperature

“…Then the core is resaturated with brine to Sw = 100%, permitting deter- To explain this anomalous behavior, we recall that an increase in asphaltene content in the tar phase of Peace River cores had been observed in previous flow work. 2 Progressive stripping of asphaltenes from the tar effluent, after transport through the cores, was also observed. This accumulation of asphaltenes could be caused by adsorption on mineral surfaces, which would be expected to result in intermediate-or oil-wet conditions, or filtration of the flowing tar through asphaltene/resin micellar gels formed at pore throats during tar flow.…”

“…Increases are consistent with results previously reported. 2 At each flowing OWR, electrical resistance and pressure drop across the core were monitored until approximately steady conditions were obtained. Electrical resistance measurement permits estimation of brine saturation with an uncertainty of ±4% and is accomplished by means of the following procedure, as described previously.2 The core is flushed with a succession of solvents to remove the tar, followed by methanol to remove the brine.…”

“…Upon resumption of flow after some intermediate time period, the initial pressure drops were generally lower than the values obtained at the termination of the preceding step. 2 2. Accumulation of asphaltenes was also observed in inputflowline sintered-metal filters where adsorption is not expected.…”