As oil and gas developments mature, reservoir depletion reduces field output and fewer opportunities exist to drill new wells. Drilling new wells as the sole means of increasing field production often becomes less profitable, and it presents greater operational risks. Economic risks are also greater as the chance of completing good wells is getting less and the higher capital investment required.
In many fields, operators, either intentionally or unintentionally, bypass pay zones during initial development by focusing only on the best zones. Accessing bypassed thinly laminated formations and low-permeability zones is economically attractive but poses several challenges.
Several techniques were used to achieve sustainable commercial production from the bypassed zones in East Kalimantan. Hydraulic fracturing and underbalanced perforations were tried, with inconsistent results. Drilling new horizontal wells was not economical.
Coiled-tubing (CT) drilling was the solution that provided a cost-effective alternative to the use of a conventional drilling rig. The advantages were a smaller location footprint, shorter trip times, ability to drill underbalanced, competitive rates of penetration, and through-tubing reentry.
Because only a few CT drilling campaigns have achieved both operational and production successes, a campaign was proposed that used conventional well design and drilling programs. Previous lessons learned worldwide were used to reduce the drilling risk and enhance the chance of success. This was especially important in drilling a deviated hole through the coal zone of the subject well.
This paper will describe three wells from the design phase through post-job evaluation. Lessons learned and improvement plans are also incorporated in this paper.
Cost-effective development of a low-permeability gas reservoir in East Kalimantan's Badak and Semberah fields (Fig. 1, 2) has proven to be difficult. Conventional production techniques have not been able to produce the reserves at commercial rates because of the small and highly compartmentalized reserves. The current production optimization techniques, hydraulic fracturing and extreme underbalanced perforating, showed inconsistent results.
A horizontal well was an ideal solution for field development, but because of the size of the reservoirs, drilling new horizontal wells conventionally was not economically attractive. Reentry drilling using existing wellbores was determined to be the best option to develop these fields, and coiled tubing (CT) drilling reentry applications were introduced.
CT drilling techniques have evolved in regards to drilling practices and CT workstring limitations. Previous CT drilling experiences in the area were not very successful mechanically, mainly because of the candidate selection process.
A joint feasibility study was performed to select well candidates that have a high potential for incremental production with minimum drilling risk. This objective was achieved with the combination of an extensive reservoir engineering study and reviewing lessons learned from other CT drilling operations [1,2].
A CT drilling reentry campaign was proposed with the key objective of maximizing gas deliverability and reserve recovery in a safer, cost-effective, and timely manner from low-permeability reservoirs and low-productivity wells. Deliverables of this CT drilling campaign were to achieve mechanical success, determine realistic costs, resolve drilling risk and evaluate low permeability reservoir productivity.
A well in the Semberah field that represents the average reservoir characteristics in the area of operation was chosen as the study case. The candidate selection process started by studying potential incremental production from a horizontal well against that of a vertical well, including considering the tubing size and its potential for production restriction.
The openhole wellbore size was simulated to evaluate the effect of friction and length on potential production. The length of the openhole segment was reviewed not just from the most likely case to the limitation of CT drilling reach, but also with geological knowledge, limited well control, additional drilling time, cost, and associated drilling risk.