During drilling of the first well, it became obvious that MPD techniques were required to lower equivalent circulating density (ECD), minimize fluid losses and prevent stuck pipe. A series of tests in the front-end loading stage was carried out, covering:
• Major projects common process.
• HSE management plan, interface plan.
• Technology plan, implementation plan.
• Basis of design.
• Core studies, geomechanics, fluid.
• Equipment modifications, integration.
Results showed that penetration angle in the shale would be critical to stability and that high build rates were required for a suitable angle.
MPD techniques were chosen over underbalanced drilling techniques because of the ultra-low permeability of the reservoir and the low likelihood that there would be productive natural fractures. This meant that injection of nitrogen or another gas would be required to lower the ECD in the hole. Specialized multiphase hydraulics modeling was performed to evaluate hole cleaning, drilling ECD, and operating envelope for the batch of wells.
The initial scope for the CTD project included window milling and drilling of the curve. During the drilling of the curves with the CTD unit, the use of bi-center bits and under-reamers put extreme stresses on the directional BHA, sometimes causing failures and nonproductive time.
A change of scope was implemented: Before the CTD unit arrived, a workover rig would perform the window milling, drilling of the curve, and landing of the flush joint liner. The CTD unit would then drill a 1,500- to 2,000-ft. lateral. Completions were carried out with the CTD rig.
Six CTD wells were drilled encompassing the full scope. With an average curve length of 450 ft and an average lateral length of 1,500 ft, the average ROP was 28 ft/hr. These six wells showed that CTD was most beneficial for the drilling of the laterals and not cost-effective for the window milling and drilling of the curves.
Overall, CTD performed 20 successful wells, averaging 1,700 ft of lateral length at an average of 48 ft/hr. CTD averaged seven days on location to perform these functions at the beginning of the campaign. As lessons were learned, this decreased to five days.
Additional information about this project can be found in IADC/SPE 122272, “Coiled Tubing MPD for Tight Gas Field Re-entry Work,” presented at the 2009 IADC/SPE Managed Pressure Drilling & Underbalanced Operations Conference & Exhibition, 12-13 February, San Antonio, Texas.