Innovative Mini-DST and Production Evaluation Approach at Challenging HPHT Condition for KeShen Tight Gas Reservoir, a Case Study at Tarim Basin

Abstract

KeShen block at Tarim basin is one of the most important gas reserve for PetroChina. The single well gas production at KeShen area can be as high as million cubic meters per day. However, the complex geology settings of KeShen area lead to significant difficulties on the engineering aspects from the drilling, logging, completion to testing. The reservoir is below a salt layer, having a nature of high pressure and high downhole temperature. Often, the reservoir section must be drilled with 6in hole or even smaller hole size. The combination of challenges: ultra-deep (more than 6000 meters), high temperature, high pressure and slim hole cause the testing operation usually lasts for a few months in time and costs a few million dollars, which is extremely expensive for a land well. Mini-DST with formation testers therefore is considered as an alternative option to providing the necessary fluid, pressure and productivity information. The main target of the reservoir sand is tight, the permeability is ranging from 0.01 mD to 1mD. Mini-DST operation was not feasible in such challenging environment with the conventional formation testers in the market. A recent introduced new equipment, named 3D radial probe fitting for the slim hole allow a feasibility trial for mini-DST in KeShen area. By the design, the new equipment can work in 6in open hole and provide max. 8000psi pressure drawdown capability. A simulation work has done to confirm this high drawdown can enable the gas flow from the formation to accomplish the testing. A numerical approach, which integrating all the logging data set and the mini-DST result, is conducted to understanding the production potential of the well. A single well numerical model is created using the logging data, the permeability is carefully calibrated with the mini-DST interpretation, the hydraulic fractures can also be added to the model for feasibility study on well stimulation. In the last 2 years, multiple wells were selected in KeShen area to do the mini-DST with the new 3D radial probe, the results were very promising. The mini-DST done in the trial wells provide high quality PVT gas samples, high quality pressure transit data for permeability interpretation. The numerical production evaluation also matches very well with the DST result later for comparison. The whole mini-DST operation takes only a few days including conditioning the mud, which is a signification saving on the time and cost. All the cases in these wells will be reviewed and discussed. With this confirmative result of the work, Mini-DST becomes the first option to test a well at KeShen area if the well condition allowed to reach optimal balance between the data obtained and operation cost.