Diagnosis of Sustained Casing Pressure from Bleed-off/Buildup Testing Patterns

Abstract

Abstract When the casing pressure can not be permanently bled off through needle valves at wellhead, the casing is said to exhibit Sustained Casing Pressure (SCP). Since high values of SCP may threaten worker safety and environmental protection, MMS regulations require remedial treatment. Testing of SCP could provide information on parameters causing gas migration and SCP. A mathematical model for testing SCP buildup has been developed. The objective of the model to identify the flow mechanism of external leak leading to SCP. The model also determines critical parameters of the leak to be used in selecting some remedial actions. Also presented in the paper are two examples of using this model to analyze actual SCP field tests. The analysis gives acceptable estimates of the depth of gas invasion zone, formation pressure, cement conductivity, and expected maximum value of casing pressure. Introduction Problem of SCP is common in the Gulf of Mexico (GOM). Recently, over 11,000 casing strings in over 8000 wells have been reported with SCP1. The US Minerals Management Service (MMS) regulations 30 CFR 250.517 require remedial operation on a well if any of its casing string has significant SCP problem. MMS has also developed guidelines to tolerate small values of SCP - a departure from 30 CFR 250.517. However, wells with approved departure must be frequently tested so that severity of SCP could be monitored and controlled. Presently, testing of SCP is mostly qualitative and limited to arbitrary criteria for casing pressure buildup. Such information is insufficient for operators to quantitatively analyze SCP problem and prevent potential risks. Thus there is a need for improved analysis that could provide information on the parameters causing gas migration and SCP. Field Data Analysis. We have analyzed casing pressure data from an offshore oilfield in the GOM2 and compared the results with MMS data from the whole GOM1. Of 26 wells total, 22 wells (85%) have SCP problems (Table 1). As indicated by the table, the following statistics may be observed:About 30.8% of the casing strings exhibiting SCP are production casing.About 65.4% of the casing strings exhibiting SCP are intermediate casing strings.About 34.6% of the casing strings exhibiting SCP are surface casing strings.About 15.4 % of the casing strings exhibiting SCP are conductor casing strings. The statistical analysis shows the trend similar to that reported by MMS1 (Fig. 1), with exception of the intermediate casing strings. The SCP problem in terms of casing pressure values can be also sized. Among casings affected by SCP, about 50 percent of the production casings and 35 percent of the intermediate casings have SCP values less than 1000 psi. For the other casing strings, about 90 to 100 percent of the strings have SCP values less than 500 psi (Fig. 2). Typical Patterns of SCP Buildup. We have analyzed historical data of casing pressure change from 38 casing string affected by SCP. Fig. 3 shows a typical pattern of SCP buildup. Of all 38 casings with SCP, 82 percent have this pattern of buildup. The casing pressure rises quickly after the bleed-off. Then, it stabilizes at a certain level. In some cases, the buildup rate may be very slow without final pressure stabilization as shown in Fig. 4. Such a low-rate incomplete buildup would be difficult to analyze. Abnormal Behavior. Fig. 5 is an abnormal case. The well was shut in at about 500 days. The casing pressure record is scatted significantly. It is very difficult to identify any obvious trends from the data.