The Kenshen tight gas field, located on the northern margin of the Tarim basin, western China, has extreme reservoir conditions of an ultra_depth reservoir (6500 to 8000 m) with low porosity (2 to7%), low matrix permeability (0.001 to 0.5 md), high temperature (170 to 190°C), and high pore pressure (110-120 MPa). Those conditions result in high completion costs and a significant difference in individual well production rates; with only one-third of wells drilled meets expectations. Previous studies focused on natural fracture(NF) and attempted to classify reservoir qualities based on the density of NF. Unfortunately, some NFs were closed or cemented by clay or calcite, and it is hard to distinguish open NF from closed NFs using well images in oil-based mud, which is widely used in this tight gas field for reservoir protection. Thereby, no positive correlation between NFs density and productions has been identified, even with the same stimulation treatment.
In this study, a comprehensive geological study was conducted to find a new way of characterizing the effectiveness of NF. First, the initial and development stages of NFs were recontructed through a tectonic activity study. Two stages were detected and showed different strikes. Second, petroleum system modeling technology was applied to simulate source rock maturation and gas migration, which revealed that gas generated in the Jurassic source rock migrated to the Cretaceous reservoir formation through faults activated in the same period as the late stage of NFs development. NFs developed earlier were closed or cemented by calcite of later deposition; those at late stage were open and effective for gas charge. Also in this study, Advanced analyses of borehole images indicated an alternative way to delineate NFs developed at different stages using geometry (i.e, crossed NFs shall include those ones developed at later stage). Parallel NFs with its development unidentified can be classified through the intersection angle of fracture strike and maximum stress direction. The smaller the intersection angle is, the easier it is for stimulation and alos the higher for the well production. Based on this study, we have divided reservoirs in the study area into three classes: class 1, reservoir with crossed NFs; class 2, reservoir with fractures of small intersection angle; class 3, reservoir with fractures of large intersection angle. This innovative reservoir classification through NF geometry is currently used in the field to determine formation stimulation method. Class 1 reservoir can benefit from acidizing alone with low completion cost. Class 2 reservoir of should be hydraulically fractured with acid. Class 3 reservoir of should be fractured with sand and proppant sand to achieve economical production.
Reservoir classification with NFs geometry had been applied successfully to guide stimulation design in the Keshen tight gas reservoirs. It is a practical and feasible way to choose the most appropriate stimulation treatment method to optimize well performance and avoid restimulation to reduce costs for this extreme type of tight gas field in western China.
Super-high pressure gas fields in Tarim Oilfield mostly lie in the area of Tianshan mountains. the upper cap rock of these gas fields is salt gypsum formation which is deeply imbedded, this salt gypsum formation contains not only many different pressure systems but also soft mudstone and salt-water bed with super-high pressure, 80 percent of all accidents and complex situations take place in this section. Tarim Oilfield arranges well Dina-11 in Dina structure in order to change this adverse situation; all results and achievements during drilling operation are as follows:
Consummate the drilling technology for Shanqian structure and reduce the possibility of accidents and complex situations, quicken the speeds of exploration and development through drilling operation in well Dina-11
Dina area as a main anticipative reservoir of Tarim Oilfield also lies in the structure of Tianshan mountains(Fig.1), as mentioned earlier. It exists great difficulty and risk in drilling operation in this area too. For this reason, Preliminary prospecting well Dina 1 and well Dina 2 arranged earlier in this area were one after the other blowout out of control for drilling accidents.
Well Dina 1, was starting drilling operation in 1999. When the well was drilled at 4440.2m with drilling fluid which density was 1.52g/cm3 on July 14th, 2000. The pump pressure rised from 17.2MPa to 38.1MPa,the safety latch in mud pump was cut. Then, the drill stem was stuck, the upper/down kelly cock and valve in standpipe could not be closed. Eventually, blowout had to performed through this rout way of kelly ?standpipe? kill line manifold? relief line ,the density of ejective brine is 1.24 g/cm3, Cl- is 199000mg/L.The salt crystal block the drill stem. For this reason, this well was abandoned with cement injection at last.
After well Dina 1, Well Dina 2 started drilling operation in 2000.When the well was drilled at 4875.59m with 1.85 g/cm3 mud on April 29th,2001.The kick occurred, after well shutdown, the shut-in casing pressure was 16MPa and the shut-in standpipe pressure was 14MPa.Afterward,the choke valve which installed in the choke manifold was opened up and discharged fluid in hole. Then, shutting in the well again, the caing pressure was 33MPa and the standpipe pressure was 27MPa. During kill operation with fracturing truck,the casing pressure went up 66MPa,the well-control equipment was failed and led to bolw out and fire. After subsequent wrecking and fire fighting operation in 66 days, kick-killing succeed finally.
Hales, John (Halliburton Energy Services Group) | Smith, Ian William (Halliburton Energy Services Group) | Wah, Kee Yong (Tarim Oil Company) | Xun, Liu Jian (Tarim Oil Company) | Yong, Li Ru (Tarim Oil Company) | Qiu, Liu Ming
Tarim Oil Company, a major producer in northwest China, operates the Kela II field in the remote province of Xinjiang. Their completion program required underbalanced perforating for several large-volume gas wells with estimated record breaking flow rates. To support the operational priorities on safety, reliability, and flexibility in the completions, options for perforating were limited to wireline-deployment and modular gun-hangar systems so that gun removal would be possible if any nonconformity occurred. To obtain the maximum well production after perforating, it was imperative that minimal wellbore damage occur. Killing the well was not a viable option since the reservoir properties in the Kela field are fluid sensitive. Therefore, killing the well after perforating underbalanced would increase near wellbore damage and produce a higher skin.
This paper will discuss the two wells completed in this field using a newly developed perforating method. There were four wells developed, but two were completed by another perforating service provider. The expected production from each well was ~4 MMscf/d. Production from the two wells completed with the methods described in this paper exceeded expectations by 46% with a rate of 5.85 MMscf/d at full open choke with a 5 MPA drawdown. The production from the two other wells that were perforated using other methods fell short of expectations with rates of less than 4 MMscf/d.