Cost Saving Through Casing Design Optimization ALA-3 as Case Study
Authors: Chineke Joseph Junior , Alabi Tunde , Okwa Hope Dafe 
Published: August 2021

First E&P commenced her development drilling campaign in the ALA field with a set of four batch wells (ALA west Phase 1A). Casing design and well architecture for the four wells were determined using standard design software, as the field had significant appraisal activities. However, actual drilling of the top-hole section on the four wells were very challenging such that, only in one of the four wells was the surface casing successfully run to depth and cemented as planned. Issues encountered included but not limited to:

Trouble drilling through very reactive gumbo clay. Trouble pulling out of hole drilling BHA. Trouble running surface casing through long section of reactive clay causing stuck pipe (Casing got stuck while running in hole). Although, a leading contribution to getting stuck was differential sticking which can be attributed to not being able to run the casing with centralizers installed (bow spring centralizers) because they got hung up at the conductor shoe which was a buckled at the shoe due to piling effects.

As a result, this study was carried out to investigate the possibility of mitigating the identified challenges, while optimizing the drilling of the top holes to improve casing and cementing operations. A starting point was to estimate the theoretical minimum casing setting depth. Using a Pressure Balance method, calculations were made to derive a mathematical model for the kick tolerance. The kick tolerance requirements were then derived in line with company policy and pore/fracture pressure information from offset wells and studies data (MDT & LOT) to arrive at the minimum casing setting depth.

A second mathematical model based on limited gas kick model load case was also derived from pressure balance calculations, to estimate casing internal pressure profile when a gas bubble reaches surface during well control circulation using drillers method. A realistic criterion for estimating burst load, consistent with the definition of kick tolerance, was then proposed, to optimize the casing design.

The study estimated cost savings of up to $1 million/well could be realized. The approach confirms huge cost savings can be realized by optimizing casing setting depths, and that illustrates the impact of safety factors.

The depth proposals from this study were like the depths planned for the new drilling campaign of phase 1A+ wells, the result of actual drilling of the phase 1A+ wells (ALA-5 & ALA-6) confirmed the following,

Surface holes in ALA field can be drilled to shallower depths. This may have shortened the time to run the casing to depth given historical challenges. The actual depths of the surface casings are like the depth proposed by this study. Problems of drilling with water-based mud can be mitigated if drilling intervals are short. A price comparison of top-hole actual drilling and running casing of ALA-3 a phase 1 well and ALA-5 & 6 phase 1A+ wells showed significant cost saving estimated above $1 million. This is like the cost saving estimated from the initial study. The surface hole drilled total depth and casing shoe depth on the phase 1A+ wells confirmed the pre-drill kick tolerance estimated in the study for these depths in the phase 1A+ wells.