RIASSUNTO
Abstract
Optimizing Workover plan and completion procedure for the Karachaganak reservoir brought superior results and created development opportunities for the reservoir. The new approach is based on rigless investigation, data integration and innovative methods to overcome different well problems.
Karachaganak field is a giant gas condensate field located in the north west of Kazakhstan. The structure is sealed with a salt layer that varies in thickness across the field. Salt that are not fully protected could creep causing casing collapse especially if subjected to negative differentials.
Initial well design applied on the 240 exiting wells imposed limitations on workovers. Additional field challenges are; severely corroded and potentially stuck completion strings, poorly cemented casing, and scale deposition.
The new approach has been developed over more than four years of operations. Successfully, workover time has been reduced by more than 25%, saving about $850,000 per workover. Well risk identified and failure risk is minimized. Well abandonment recognition, planning and cost have been optimized.
Introduction
Karachaganak Field is a giant near-critical gas condensate reservoir located in northwest of Kazakhstan near the Russian border as presented in Figure 1. The field was discovered in 1979 and began limited production in 1984 under the operatorship of Karachaganakgazprom, a subsidiary of Russia's GazProm. In 1992, when Kazakhstan became an independent state, Kazakhstan state gas company, Kazakhgas, took over operatorship. In November 1997 a Production Sharing Agreement (PSA) was signed by the Republic of Kazakhstan, Agip, BG, Texaco and Lukoil. In 1998 Karachaganak Petroleum Operations (KPO) was formed and began operating the field.
Wells drilled during initial field development were targeting combined gas and condensate production. This objective according to the PSA has changed to be; accelerating early oil production, increase long term gas condensate production and re-inject 40% of the gas to optimize the field recovery. Consequently, the 240 wells that were initially drilled had to be workedover and recompleted to achieve the change in the development objectives.
This paper will describe improvements in the workover approache and utilized equipment to overcome various operation difficulties faced in old wells workover. Field geology and reservoir description will be presented pointing out the various operation difficulties. The optimization to reach well objectives with more successful technical approach will be presented in two categories, rigless pre-workover preparation and rig workover optimization. The technical and economical advantages would be illustrated through typical examples of workovers comparing the new approach to the conventional one.
Geology and Reservoir Description
The Karachaganak field structure is a carbonate massif that consists of heterogeneous reef and platform carbonate complex measuring 30 km by 15 km. The field is located on the northern margin of the pri-caspian basin that contains a number of major hydrocarbon discoveries in the area such as Astrakhan, Tengiz and Kashagan. The Karachaganak reservoir has been found to be highly heterogeneous and the limestone deposits is of low porosity and permeability with local enhancement due to dolomitisation. The dolomitisation has been adversely impacted by the early diagenetic marine calcite cementation and later diagenetic anhydrite precipitation. The top structure map of the field is presented in Figure 2. The reservoir ranges in age from the Late Devonian to the early Permian. The structure is sealed with Permian evaporates salt layer that varies in thickness across the field.