The high dip angle in formations is one of the reasons makes it difficult to drill in Tarim Basin, resulting in much longer drilling time. To counteract the high pressure from the salt layer above the targeted gas zone, 16-mm heavy casing and drilling mud as heavy as 1.9 g/cc must be used. As drill bit and pipes continue repeatedly tripping in and out the well the heavy casing may get wore thinner. Therefore it's important to know the degree of the casing wear, and to recalculate the remaining strength. Furthermore, if channel in cement behind the casing exists across the high pressure layer, it will pose another threat to corrode the casing from outside and reduce the overall strength of the casing. Hence the cement quality behind the heavy casing must be measured and assessed by reliable means. The tradition technology for well integrity evaluation has limitations in providing answers to the casing corrosion and zonal isolation in such challenge downhole condition. A new generation of the ultrasonic imaging tool was introduced to increase the certainty of casing mechanical property measurement and cement evaluation. The technique combines the pulse-echo ultrasonic measurement with a new flexural wave measurement. The analysis from this combination allows not only a better discrimination between solid, liquid and gas for cement evaluation behind casing, but also more accurate casing information including internal diameter and casing thickness at high resolution in one trip in a well. A logging example was illustrated in this paper with high quality logging data acquired in difficult down hole conditions. As can be seen in this example, many casing grooves were found, and metal loss were up to maximum 4mm at some spots. This paper also shows a new way to look at cement quality behind the heavy casing.
Formation testing has been utilized widely worldwide as well as in China for field exploration and appraisal, helping reservoir characterization and evaluating hydrocarbon reserve. However, using formation testing technology at development stage in order to improve the reservoir ultimate recovery is not mentioned much in published literatures. In this paper, two case studies were summarized on how formation testing was used at the secondary and tertiary stage of development field in west China including the first domestic gas injection monitoring application.HD Field is one of the oilfields of PetroChina which contributes significant oil production in Tarim basin. With strong aquifer support, one challenge in the adjustment of oilfield development plan is to identify how a potential zone has been water flooded. This case study demonstrates how data acquired from the formation testing helps reservoir engineering study, revises the field development plan and eventually identify the new wells locations with encouraging production result.The second case study demonstrates the first ever application in China of gas injection monitoring in field YH. This field is rich of gas condensate and utilizes gas injection as the main approach to drive the field production and to maintain the reservoir pressure. Because of commingle gas production from multiple zones; it is challenging to understand the sweeping efficiency of injected gas from different formation. With the help of real time downhole fluid composition measurement and innovative new designed probes, formation testing is able to pin point zones that have been swept by injected gas. So the gas injection strategy was adjusted accordingly to improve the overall field recovery.
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.
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