Compositional Gradients in the Anschutz Ranch East Field

Abstract

Summary. Experimental data from analyses of more than 20 recombined fluid samples show that a significant compositional gradient exists in the Anschutz Ranch East field. The gradient can be correlated as a linear function of depth. Using the correlation, an engineer can estimate compositions for any bottomhole location and then use other methods to estimate fluid properties and/or depletion performance. Introduction The Anschutz Ranch East field is located in the western Overthrust Belt along the Wyoming/Utah border (see Fig. 1). Initial surface production from the west lobe of the Anschutz Ranch East field production from the west lobe of the Anschutz Ranch East field gave the erroneous indication that this reservoir contained an oil either underlying or separate from other zones containing gas condensate-type fluids. The discovery well tested four different of perforations covering more than 600 ft [183 ml of structure in perforations covering more than 600 ft [183 ml of structure in the Nugget formation. The lower interval was initially thought to be an oil because it produced a stock-tank liquid of dark color and about 4]'API [0.82 g/cm3] gravity (specific gravity= 0.81948). Laboratory PVT studies, however, showed that all four sets of perforations were producing rich gas-condensate fluids with different perforations were producing rich gas-condensate fluids with different compositions, GOR'S, and saturation (dewpoint) pressures. These first few samples were analyzed without benefit of any geologic data regarding the relative isolation/communication of the four zones. Delineation wells showed that the structure continued above the perforated intervals in the discovery well. Assuming that the perforated intervals in the discovery well. Assuming that the reservoir was continuous, we estimated the saturation pressure near the top of the structure (by extrapolation of the available saturation pres-sure data) and found a dewpoint greater than the average reservoi pres-sure data) and found a dewpoint greater than the average reservoi pressure at that depth, which was not possible in this case because pressure at that depth, which was not possible in this case because deeper fluids were undersaturated gas condensates. It is possible for a recombined sample to have a saturation pressure greater than the reservoir pressure, but this indicates a situation in which two phases (free reservoir gas and saturated reservoir liquid) were both phases (free reservoir gas and saturated reservoir liquid) were both flowing together into the wellbore at the time the surface separator GOR was measured and separator fluid samples were collected. Recombination in these cases results in either too much solution gas (for a black oil) or too much retrograde liquid (for a gas condensate), both of which usually raise the saturation pressure. It was also determined from geologic data that we were dealing with one very thick zone rather than several isolated ones, supporting the concept of a continuous reservoir. According to the results of the PVT tests, it seemed highly unlikely that an oil leg was present. We were therefore forced to explain the problem of an apparent two-phase fluid system (extrapolated saturation pressure above reservoir pressure) located in a continuous reservoir at an elevation above a rich gas-condensate system. At this stage in the field's development, it was jointly decided between the research and producing departments to initiate a large-scale data-collection program on all future wells drilled in Anschutz Ranch East. The purpose of this program was to evaluate fluid properties from all wells and to develop an understanding of how those properties from all wells and to develop an understanding of how those properties varied with depth in this 1,800-ft [550-m] -thick reservoir. properties varied with depth in this 1,800-ft [550-m] -thick reservoir. Data Collection For the next 2 years, multiple sets of surface separator gas and liquid samples for recombination were collected from every well drilled in the Anschutz Ranch East field. These were transmitted to our research center for PVT analysis. In several cases, duplicate samples were sent to service company laboratories for corroborating analyses. The suite of information on each well, which was used for correlating purposes, was as follows.Well test information-true vertical depth (TVD) subsea, reservoir temperature, producing surface separator gas/liquid ratio (GOR), separator temperature and pressure, and stock-tank liquid gravity.Laboratory analyses-composition of separator gas and liquid samples and recombined wellstream through C6 with a lumped C7+ fraction, detailed chromatographic analysis of the C7, fraction, molecular weight and specific gravity of the lumped C7+ fraction, density of recombined fluid above the dewpoint pressure, dewpoint pressure of the recombined fluid, constant-composition volumetric expansion (CCVE) data (including retrograde liquid behavior), and variation of dewpoint pressure and CCVE with temperature for selected samples. The PVT analyses (CCVE tests) run at the research center were conducted in variable-volume visual cells similar to the ones described by Jacoby and Yarborough. Samples for chromatographic analysis were obtained with the procedures described by Yarborough and Vogel. Densities were obtained through use of high-pressure pycnometers. Replicate samples were obtained for compositional pycnometers. Replicate samples were obtained for compositional analyses as long as sufficient fluid was available in excess of that required for other tests. Compositions were determined with a dualchannel gas chromatograph that had an inlet splitter and used Chromasorb P-101 and SE-30 columns for the thermal conductivity and flame-ionization channels, respectively. Simulated true-boiling-point analyses by temperature-programmed gas chromatography (also referred to as "fingerprint" or "type" analyses) were performed on the lumped C7+ fraction with a 150-ft [46-m] capillary column. This test provided reasonable definition of individual carbon number cuts through carbon number 40. Data Correlation Compositional analyses of the fluids used in the correlational work are provided in Tables I and 2 (Table I for those analyses performed by our research center and Table 2 for those analyses provided by commercial service companies). Also included in these tables are the specific gravities and molecular weights of the lumped C7+ fractions, the GOR's corrected to a set of standard separation conditions (500 psia and 80 degrees F, 42 psia and 70 degrees F, and 12 psia and 60 degrees F [3450 kPa and 300 K, 290 kPa and 294 K, and 83 kPa psia and 60 degrees F [3450 kPa and 300 K, 290 kPa and 294 K, and 83 kPa and 289 K]), the corresponding depths from which the samples were collected. Because the production from the Anschutz Ranch East field was all from one formation (Nugget), it was considered highly probable that the hydrocarbons all came from the same source material. If so, the distribution of the heavy-component cuts within the C7+ fraction for all the sets of recombined fluids would be expected to be relatively similar (on a normalized basis). This assumption was found to be valid for west Texas oils, as reported by Chaback and Turek. SPERE P. 1025