Research and Practice of Full Life Cycle Well Integrity in HTHP Well, Tarim Oilfield

Abstract

There is abundant natural gas in Kuqa foreland area of Tarim basin, the reservoir has characteristics of ultra depth(6500m-8000m), high pressure(115MPa-140MPa), high temperature(170°C-190°C) and complex corrosive medium, which bring well integrity high challenge. There are 52 production wells in the Kuqa foreland basin in 2012. The 16 wells of these production wells exist sustained casing pressure(SCP). The SCP wells caused by the tubing string failure is 61%. Since 2012, in order to build safe well barrier and achieve scientific production management, consider all important periods of drilling, well testing, well completion and production in well full life cycle. the design methods of the four main well barrier components(casing string, cement, tubing string, well head) are optimized. A set of technology for well barrier quality control, well maintenance and well production management is formed. Finally, the key technology of full life cycle well integrity for ultra depth, HPHT gas well is formed. In view of the large angle of bedding, well deviation controlled difficultly, and coexistence of high pressure saline aquifer and week bed in one open hole section which lead to serious casing wear and poor cement stone, the vertical well drilling technology, the collapse resistance casing design methods, the casing wear preventing measures, negative pressure test method and high temperature, high density cementing technology are completed. These technologies improve the safety of casing string and the quality of cement. Considering the extreme working conditions(the partial pressure of CO2 is beyond 2MPa, The content of chlorine is about 80000mg/L, acid job, 136MPa pump pressure in well head)of tubing string, material selection method, triaxial stress analysis method, quality control technology for tubing string is optimized. The failure ratio of tubing string reduced from 50% in 2013 to zero in 2017. Referencing API RP 90-2 and IOS16530-2, a special calculation method for annulus pressure management is formed, which include minimum operation pressure and maximum allowable pressure for different annulus. Then a risk assessment system is established. The SCP wells is controlled less than 22% in the condition of increasing number of high pressure gas wells. Based on the research introduced above, China's first set of well integrity specification series is completed. The integrity specification series include the well integrity guide, the well integrity design and the well integrity management for high temperature high pressure and high sulfur gas. These results effectively support safty in production for high pressure gas field in Tarim basin, and will promote the well integrity level in the same kind of oilfield.

Experiment and failure mechanism of cement sheath integrity under development and production conditions based on a mechanical equivalent theory

“…1,2 The problem of sustained casing pressure (SCP) is more serious there because this oilfield is directly affected by burial depth (nearly 7000 m), high formation pore pressure (more than 120 MPa), and harsh working conditions; these factors considerably increase the maintenance cost and production risk. 3,4 Regarding the 14 wells in Kuqa Piedmont, we found that the abnormal pressure values and ratio of the A-annular spaces were relatively high statistically (Aannular spaces refers to the annular space between the tubing and the production casing); further, some wells had multiple annular spaces with abnormal pressure, as shown in Figure 1. The analysis results indicated that in addition to the effect of primary cementing quality and casing damage, the failure of cement sheath integrity because of stress damage was another important cause of SCP.…”