Abstract

It was difficult to drill and complete the wells in the Kenkiak Under-Saltdome oilfield in Kazakhstan due to complicated geological conditions. Among 42 wells which had been drilled by former operator 40 wells were abandoned; amid 40 abandoned wells 12 were engineering abandoned; only 2 wells were capable of production with 38 to 69 tons of oil per day. Because of low drilling rate of success, long cycle of drilling, and low production, the former operator regarded the field an unable to be developed one. Consequently the field had not been put into production since its discovery 30 years ago.

In order to increase the productivity, near balance drilling and open hole completion have been accepted for vertical wells in the beginning in high pressure low permeability fractured carbonate reservoir. The first trial met with success and initial day rate of production was as high as up to 480 tons of oil. Subsequently, near balance drilling and open hole completion have been adopted for other vertical and high-angle wells, average day rate of production exceeds 300 tons per well.

However, a few wells completed open hole encountered tubing blocked when put into production, which resulted in those wells off production. To prevent the wells from tubing blocked during putting into production, as well as to maintain high productivity, subsequent wells that are not suitable for open hole completion have been liner completed. Average day rate of production of those wells completed with liners also exceeds 300 tons per well.

Production of some wells completed open hole or liner exceeds 1000 tons per day per well. Open hole completion and liner completion have greatly increased productivity as compared to those wells completed perforation, turning it from an unable to be developed marginal field into one with annul capacity of 2 million tons of oil.

Background

The Kenkiak Under-Saltdome (KUS) oilfield was discovered by former USSR in 1971. The geology of the field was so complicated that the drilling and completion were hard to be accomplished. The complicated geological conditions could be reflected in, but are not limited to, the following aspects:

  • The salt, gypsum and interbeded plastic mudstone in the Permian were subject to swelling and creeping thus resulting in stuck pipe and crush casing.

  • Gas cut from the shallow-layer hi-pressure gas formation lain in the salt dome could lead to blowing out.

  • The lower Permian and the Carboniferous, belonging to hi-pressure reservoirs, with active oil and gas, near formation pressure and lost-circulation pressure, and narrow gap of safe density of drilling fluid, were subject to lost circulation and blowing out at the same time, or stuck pipe resulted from wall collapse.

  • The extremely thick mudstone layer interbeded with glutenite and hard mudstone in the non-saltdome upper Permian series could exert a great impact on safe and quick drilling in.

The average well depth of the Permian reservoir is 4183m, while the average drilling cycle is 24.62 rig-months; the average well depth of the Carboniferous reservoir is 4543m, while the average drilling cycle is 33.73 rig-months. Frequently occurred complex and accidents, low rate of penetration and high cost of drilling, especially engineering abandoned 12 wells impeded the achievement of deep targets and severely restricted the exploration and development of the KUS reservoir.

Introduction

An investigation revealed that out of 36 wells drilled by former operator in oil zones of the KUS reservoir, 26 wells (accounting for 72.2%) encountered oil and gas invasion and blowout; 13 wells (36.1%) circulation loss; 13 wells (36.1%) wall collapse, hole shrinkage and stuck pipe. 28 wells in total incurred the above complicated conditions, accounting for 77.8%. A weighted drilling fluid was used to balance the high formation pressure, over balance drilling (OBD) was utilized.

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