Mineral Scale Control in a CO2 Flooded Oilfield

Abstract

Abstract This paper presents a case study of improving scale inhibition treatments in the Sharon Ridge Canyon Unit (SRCU), West Texas in response to the increased scaling conditions that has arisen from carbon dioxide (CO2) injection into the oil reservoir. In anticipation of increased scale deposition, a proactive approach was taken by carrying out a comprehensive field system analysis and scale survey prior to the commencement of CO2 flooding at SRCU. This analysis/survey was intended to assess the new situation and come up with a cost effective scale treatment program. A laboratory evaluation of a number of scale inhibitors was then carried out. Based on our understanding of the problem and the lab evaluation results, a new scale inhibitor and a new chemical treatment program were developed and implemented in the field. This paper describes the system analysis/scale survey and highlights its importance in understanding the changing situation and the new challenges in scale inhibition arising from CO2 flood. The laboratory evaluation for identifying the most suitable and cost-effective inhibitor is then briefly described. The paper then discusses the new scale inhibition treatment program and scale monitoring program. The field results from implementing the new treatment program are presented and discussed. Owing to the proactive approach and the improved inhibitor and treatment, the field production system and water re-injection system have not been troubled with scale deposition. Introduction Carbon dioxide gas flooding of oil reservoirs on its own or by water-alternating-gas (WAG) is a widely use tertiary recovery method in the onshore US oil production1–3. CO2 flooding has mostly been effective in improving oil recovery from high watercut, mature oilfields1–5. However, CO2 flooding may exacerbate the existing production problems and/or create additional production problems6–10. One of such problems is calcium carbonate scale formation7,9,10. The Sharon Ridge Canyon Unit (SRCU), is located 18 miles southwest of Snyder, West Texas. Production is from a depth of 6500 ft in the Pennsylvanian Canyon limestone. This field unit has been in production since 1955. ExxonMobil (formerly Exxon Company, USA) took over operation of the unit from the original operator in 1988. By 1998, the average watercut had risen to 97% and the oil production had declined to 900 bbls/D. In order to increase oil recovery from this mature reservoir, a CO2 injection project was initiated, which resulted in the commencement of CO2 WAG injection in early 1999. The first breakthrough of CO2 in the producing wells was detected in one well within days and in the other wells after two to three months. Currently, the field is producing 2800 bbls/D oil, representing a significant production increase from the pre-CO2 production level. As expected, all of the CO2 breakthrough has occurred in the wells within the Tank Battery 5 (TB5) production area. The CO2 content in the produced gas from CO2 impacted wells at Battery 5 is 72% on average. Prior to CO2 injection, SRCU had a moderate calcium carbonate (calcite) scaling problem in the surface production and injection systems, which was adequately controlled with a simple chemical inhibition program. With the commencement of CO2 injection in early 1999, calcium carbonate scaling potential has risen considerably, due to a two fold increase in bicarbonate concentration in the produced water. The increased calcium carbonate scaling tendency, if uncontrolled, would have an adverse impact on the surface production system and the produced water re-injection system.