Kenkiyak sub-salt oil field is situated in the eastern edge of Kenkiyak tectonic belt of the Pre-Caspian Basin located in the Western Kazakhstan. The structure is a typical dome short anticline with the Permian salt dome Valley bands at the core of it. Since the reservoirs of the oilfield are buried deep with high pressure and extremely complicated geological conditions, drilling is extremely difficult, which made the field one of the well-known regions in the former Soviet Union for its drilling difficulty. The main geological features are: the diagenesis of the upper Mesozoic stratigraphy are bad; the Konggu group under the Permian developed thick salt dome, with the biggest thickness at the main site of salt dome reaching 3200 m; the buried depth of Carboniferous reservoir is between 4300-4500 m; the pressure coefficient of Permian and Carboniferous are respectively as high as 1.79-1.84 and 1.90-1.94; the leakage and kicking usually happened in the same layer; the oil and gas are rather active. The above-mentioned geological and reservoir features led to the following main problems in drilling and completion operation:

  1. Downhole complex situations happen frequently in the upper Mesozoic strata (easy to leak, easy to collapse and easy to bits balling etc.); the possibility of salt dissolution, hole shrinking, plastic flow of dome and salt re-crystallization existing in Konggu group of Permian led to the risk of the wellbore collapse, pipe stuck, casing collapse, as well as the deterioration of drilling fluid performance.

  2. The pressure bearing capacity of Permian and Carboniferous reservoirs are low, with the safe drilling fluid density window comparatively small. Within the whole construction cycle of a well, the high-density drilling fluid drilling cycles accounted for the entire 1/3 or more.

  3. The casing program of a well was complex, usually as many as 4∼5 layers of casings;

  4. Drilling cycle was long. In the former Soviet Union time, the shortest cycle of a well was 9 months, a maximum cycle of 66 months, and the average drilling cycle around 26 months.

  5. A well with perforated completion was of low yield, the initial production only 30 tons / day during the former Soviet Union time.

In the 30 years of drilling history in The former Soviet Union time, such complicated accidents as well blow out, leakage, collapse and sticking occurred very frequently. Of the 43 wells drilled during that time, 16 wells have been scrapped due to engineering reasons; 26 wells had a serious invasion of oil and gas, or a gas kick, or a blowout; Well leakage happened in 13 wells; severe sticking because of the collapse and hole shrinking appeared in 13 wells. Wells with complexity of accidents accounted for 82% of the total. The sub-Salt oil field is considered as an economic marginal oil field.

After CNPC took over the post in 2002, in view of the technology questions existing in Kazakhstan Kenkiyak sub-salt oil field in aspects of geological structure, drilling and completion, the following solutions are put forwards: 1) Optimize the casing program, to scientifically and rationally simplify the casing from 4 layers to 3 layers; 2) use the zwitterionic polymer mud system and supporting Series technology, to solve the problem such as the safety of drilling in salt, the oil and gas leak-proof and reservoir protection, the control of high-density salt water drilling fluid rheology, the anti-sticking of high angle directional well, etc. 3) adopt open hole completion combined with screen pipe completion. 4) Choose PDC drill bits, optimizing the bottom hole assembly and drilling parameters under different methods.

As of the end of 2008, CNPC had successfully drilled 55 producing wells, with the success rate of 100%. The average drilling cycle had been shortened from more than 700d of the former Soviet Union to about 180d of CNPC. Single well initial production had increased from the original more than 30 tons / day in former soviet union time to an average of 200 tons / day or more nowadays, in which many wells performed the initial output of 500m ^ 3 / d or more, one well with the initial daily output of 1000 tons. Casing simplification, rational drilling fluid system, the bits selection and the open hole completion Ways and salt are key factors of the sub-salt field with high yield.

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