Investigation of Increased Gas-Oil Ratios in Ain Dar Field
Authors: Al-Eid Mohammad I. , Kokal Sunil L. 
Published: December 2007

Reservoir and production engineers have observed an increase in gas oil ratios (GOR) with increasing watercuts in north Ain Dar (ANDR), away from the gas-cap area. Several of these wells now show GORs approaching 1,000 scf/stb. The original GOR in this area is ~570 scf/stb. This variation in GOR has a direct impact on wellbore hydraulics and on the well production rates. A study was initiated to investigate the cause(s) of this abnormal increase in GOR in this area.

One hypothesis for the increased GOR is that the excess gas is being produced from the brine. To ascertain this hypothesis, both subsurface and surface oil, water and gas samples were collected and analyzed. Samples were collected from several wells with different (high) water cuts and from the gas-cap area. The analyses included the determination of gas oil ratio (GOR), gas water ratio (GWR), fluid compositions and material-balance calculations. While brine as a possible source of excess gas was the prime focus of this project, other sources such as gas-cap leak off, carbon dioxide solubility in water, gas metering problems were also be investigated. The results of this study indicate that the gas-oil ratio increases with increased watercuts. However, the increase is not enough to explain the values observed in the field. One possible suspect is gas metering with orifice plates. These possible causes of increased GORs will be highlighted in the paper.

Introduction

Ain Dar field is part of the giant Ghawar oil field. There are six gas oil separation plants (GOSPs) in Ain Dar field processing Arab-D (Arab Light) crude. Since the early 1990s, Abqaiq Reservoir Management and Production Engineering have observed an increase in gas oil ratio (GOR) in the Arab-D reservoir with increasing watercuts in northern Ain Dar, GOSP-2 wells (Figure 1). Several of these wells show high GORs, some exceeding 1,000 scf/stb. The original GOR in Ain Dar field is ~570 scf/stb. This variation and increase in GOR has a direct impact on wellbore hydraulics and on well production rates. Possible causes of the increase in GORs include gas evolving from brine, a lateral communication with the gas-cap gas, leakage from deeper reservoir(s), or some other phenomenon. The investigative research study was initiated to determine the causes of increased GOR in northern Ain Dar area. The study explored probable causes including:Lateral communication between the gas-cap and GOSP-2 wells. To validate this hypothesis the gas-oil ratios of several wells were recorded between the gas-cap and GOSP-2 wells. Furthermore, the distance between the gas-cap and the affected area was measured to ascertain if fluid communication could be possible between the high GOR wells and the gas-cap region.Leakage from a deeper reservoir, for example Khuff. To investigate this possibility, isotope analysis were performed on several gas samples from the Arab-D and Khuff reservoirs to check if there is any communication.Excess gas being produced from the brine. To validate the gas-from-brine hypothesis, several experiments were conducted with bottomhole and surface samples to determine oil and water compositions as well as gas-oil ratios, gas-water ratios. Subsurface water and oil samples were collected, and oil, water and gas samples were collected at the separator (surface).Metering problems and hence incorrect GORs reported. During initial communication with Abqaiq Reservoir Management and Production Engineering it was stressed that the meters were working properly and the problem was not related to the meters or metering procedures. Based on this information the study focused on the first three probable causes outlined above.

Besides exploring the probable causes mentioned above, a literature review was also conducted to evaluate the solubility of hydrocarbon and non-hydrocarbon gases in water and brine. In addition, new oil composition data were also obtained and compared with previous data to ascertain if fluid compositions have changed with time.