Well Testing in Challenging Environments: A Case Study from Onshore China

Abstract The Pugang gas field situated in the Sichuan province near the city of Dazhou is one of the biggest inland gas discoveries of the last decade in China. With significant reserves, it is an important and strategic field for the operator and the region. However, with H2S concentrations as high as 10%, surface and downhole facilities design and execution still remains a challenge. Testing these wells has also proved difficult due to the surface risks inherent in a conventional testing operation. Also, with approximately 500 m of perforations, it is critical to identify the individual contributing zones before any meaningful analysis of the test can be done. Well testing using production logging tools has recently been introduced in this field and it has proven to be of tremendous benefit to the operator in terms of both identifying the inflow zones and also in performing well test analysis. Production logging was done in two phases, namely shut in and flowing passes to identify cross flows (if any) and to identify potential inflow zones. Once this is established, a buildup analysis is done to measure the productivity potential and the average reservoir pressure. A good pre-job design and real-time analysis of the data was made to avoid unnecessary exposure of the logging tools to this highly corrosive environment. This paper illustrates the challenges in planning and executing a production logging operation in challenging conditions and also shows the utility of performing these operations. Field Introduction The Puguang gas field, discovered in 2002, is the second largest gas field in China and the largest sour field discovered in the last decade (Ma, 2005). This field is located north of Chongqing in the Sichuan basin (Fig. 1). The giant field is a combination structural-stratigraphic trap closed by lateral depositional change and fault closure. The exploration area is a challenging environment, as it is located in a mountainous region with high surface elevations (up to 1,200m). The thickness of producing formations is between 176.4m~838.8m, and 395 on average. The reservoir fluid in this field is dry gas with high H2S (14.4%) and CO2 content (Ma 2007) (9.91%). The field is flanked on the west with a GWC at 5140 m TVDSS. With no voidage replacement mechanism in place yet, the production of water due to the contact movement is also a concern for the operator. Therefore production logging as a reservoir monitoring and surveillance tool has been introduced since 2010 and more than 18 wells have been tested to date.