| Description |
Executive Summary: This report describes work done by the United States Department of Energy's Laramie Energy Technology Center (LETC) from 1971 through 1982 to develop technology for future recovery of oil from U.S. tar sands. Work was concentrated on major U.S. tar sand deposits that are found in Utah. Major objectives of the program were as follows: - Determine the feasibility of in situ recovery methods applied to tar sand deposits. - Establish a system for classifying tar sand deposits relative to those characteristics that would affect the design and operation of various in situ recovery processes. Most of the world's supply of tar sand is found in the Western Hemisphere, and deposits in Venezuela and Columbia are estimated to exceed 1 trillion barrels. Canadian deposits are estimated at 967 billion barrels in the Province of Alberta. The U.S. tar sand deposits are estimated to be over 53 billion barrels, with over 20 billion barrels found in Utah. Estimates have been made that only 10% of the U.S. tar sand can be recovered by surface mining, and 90% must be recovered by in situ methods. Early work at LETC included physical and chemical characterizations of Utah tar sand. Tar sand is bitumen-bearing rock with an in-place viscosity exceeding 10,000 centipoises at reservoir temperature. Athabasca (Canadian) tar sand is a water wetted sand and therefore is amenable to water-based extraction processes, while many U.S. tar sand deposits are oil wetted sands that require different approaches to oil recovery. The U.S. tar sand typically contains 5 to 10 wt X bitumen. Uintah Basin, Utah bitumen is low in sulfur content (less than 1%) compared to Athabasca and o ther U . S . b i tumens. Laboratory work at LETC was conducted to gain insight into methods for reduction of viscosity necessary to better recover bitumen from Utah tar sand. These experiments showed that reverse combustion could be used to open and heat a flow path from a well bore into a tar sand deposit. Air flux rates necessary to sustain combustion were determined and oil yields were shown to be encouraging. A mathematical model of the reverse combustion process was developed and used to design other field experiments. Later laboratory work was also conducted with steam injection into Utah tar sand samples. Results showed that the mechanisms associated with a hot water flood are part of the steam drive recovery mechanism. Steam drive recovery of oil was also shown to have the added advantage of a solvent extraction mechanism working to increase the recovery of oil over that which would be expected from a hot,water flood. This work, combined with a steam drive simulator, was used to design and operate one field experiment described later in this report. The site selected for field experiments was located 4 miles west of Vernal, Utah, on property owned by Standard Oil of Ohio (SOHIO). This site was located on the Northwest Asphalt Ridge Deposit where tests were conducted in the Rimrock Sandstone Member of the Mesaverde Formation at depths of 300 to 500 feet. The Rimrock Sandstone is a highly saturated, semi-consolidated to consolidated, fine-grained sandstone with claystone, siltstone, and shale intervals. At the test site the Rimrock Sandstone contains continuous tar sand sections varying in thickness from less than 1 foot to more than 40 feet. The Rimrock Sandstone is heavily faulted in this area and dips steeply to the southwest at angles anywhere from 10 to 45". Core analyses and logging techniques were used to evaluate reservoir properties of proposed test locations in the deposit. Core analyses were initially used to determine porosity, oil and water saturation, and permeability for the design of the experiments. A gamma, sidewall neutron, density, SP, induction and caliper log suite were utilized in determining porosity and water saturations. A sonic log was used to determine elastic rock properties for design of a hydraulic fracture. Carbon/oxygen logs were used to measure oil saturation before and after recovery tests. The best porosity values were determjned from the density log. Formation water resistivity was estimated from the Archie equation using core porosjty, water saturation and the induction log formation resistivity. Water saturation was acceptable, but it did not match core data as well as desired. Oil saturation values from the carbon/oxygen log may he conservative because of the lack of a reference unsaturated clean sandstone. Associated environmental research characterized the emissions generated from the field experiments and aided in developing a technology for controlling those emissions. Emphasis in environmental research was on water cleanup technologies that would make process water available for use as steam. Generally this work showed that waters from in situ tar sand recovery experiments can be cleaned up using existing technology, and biological oxidation of process water was shown to be an effective treatment option. Actjvated sludge treatment was found to be an effective method for reducing the organic content of process water. The TS-1C field experiment (the first one) was conducted in the fall and winter of 1975. The objectives for this test were to demonstrate the feasibility of reverse combustion as a method of recovering oil from a tar sand deposit and to gain experience with heavy oil recovery equipment. Combustion was initiated in the middle wells of the unenclosed line drive pattern at a depth of 300 ft. It was necessary to inject air at pressures higher than lithostatic pressure to obtain the desired injection rates. Produced oil was not cracked as much as had been hoped or anticipated and plugging of surface recovery equipment became a major problem. Combustion was sustained for 63 days before the experiment was terminated after it was concluded that I.ittle additional information would be gained from continued operation. A total of 60 barrels of heavy oil were recovered and approximately 84 percent of the injected air was lost to the formatlon. It became apparent that better provisions for handling heavy oil would be needed in any future experiments. For the second field experiment, TS-?Cy the test pattern was lajd out along the strike line of the formation to take advantage of directional permeability to increase injection rates and minimize air losses to the formation. Recovery equipment was provided with heating capability to handle heavy oil. The experiment was designed to use a reverse combustion phase to prepare the tar sand zone, followed by forward combustion, which was supposed to drive the bitumen to producing wells. This second experiment was ignited in August, 1977, and continued for 183 days. Problems were encountered with production equipment failure , wellbore plugging, and formation heterogeneity. Propagation of the reverse combustion front did not occur as a distinct continuous phase; rather it occurred as several echoings of two combustion phases, both reverse and forward within the same area. Recovery of iniected air averaged 49% for the experiment. The experiment produced 580 barrels of oil, or 25% of the original oil in place. Volumetric sweep efficiency was calculated to be 86% for reverse combustion and 33X for forward combustion. This second field effort was considered to be a successful experiment and provided encouragement for further improvement in the reverse-forward combustion technique. A hydraulic fracture was tested in 1978 as a possible means of increasing permeability within a highly saturated tar sand zone. The fracture was designed from data collected from oriented core and from log-derived elastic properties. Real time evaluation of the fracture growth was monitored by surface tiltmeters. Post fracture evaluation was conducted from wells drilled to intersect the fracture and included air flow tests, TV logging, sonic logging, and casing cement bond logging. It was concluded that the fracture was dominated by a nearby fault and a zone of high permeability within the conglomerate that was on top of the tar sand zone. The fracture did not achieve the size or orientation desired due to the heterogeneity of the site. A two-well steam injection test was conducted in 1979, and this test showed that steam injection rates sufficient for a steam drive could be achieved. It also showed that bitumen could be mobilized with hot water. On the basis of this two-well steam test, a steam-flood field experiment was planned with the objectives of 1) determining the technical and economic feasibility of using a steam-flood as an in situ recovery technique in a Utah tar sand deposit, 2) evaluating an injection well completion usjng a high temperature packer, 3) evaluating several production well completion schemes, and 4 ) determining recycle and fuel use possibilities for produced water and oil. The steam-flood experiment, TS-lS, was designed using data from previous experiments, laboratory studies, computer modellng, and the two-well steam injection test. The well pattern selected included two concentric, inverted five spots, one on one-fourth acre, and the other on one-tenth acre sites. The steam-flood test (the third field test in this series) began in April, 1980, and continued until late September, 1980. A 45-foot thick interval of tar sand located about 500 feet below the surface in the Rimrock Sandstone was the target area. The steam front was observed to move preferentially down dip, and some steam was lost to the overburden through faulty casing cement. A major portion of the injected steam was lost to the underburden. Both heavy oil and light oil were recovered during the test. The overall steam efficiency for the pattern was about 16 percent. A small portion of the 1,150 barrels of oil produced was light oil, probably produced by steam distillation. Major oil production, similar in properties to the original bitumen, resulted from hot water displacement. A fourth field experiment, TS-4, was designed in late 1981 and early 1982. This experiment, planned for the SOH10 property, was planned to use the best features of in situ combustion to preheat the reservoir and then steam-flooding to produce oil. Data from laboratory and field experiments were used in computer model runs to evaluate a series of alternative design conditions. Generally, the model runs showed that if a 10-foot interval of high permeability were preheated by reverse combustion, and a steam drive was then applied to the 65-foot tar sand zone, about 40 percent of the original oil-in-place could be recovered. The model runs also showed the importance of high lnjection rates to increase the rate of oil recovery and to reduce heat losses. This field experiment was planned but never conducted. This report covers i n detail all field experiments conducted by LETC in Utah, as well as the geology and characterization of some Utah Tar Sand deposits. Environmental research done in parallel with the field experiments, as well as an economic evaluation of in situ oil recovery from tar sand, are also presented. |
