Abstract

Underbalanced drilling along the Gulf Coast recently brought new life to an old field. Natural gas misted with diesel was used to drill the low permeability Smackover and Norphlet formations at the Hatters Pond field north of Mobile, Alabama. The success of the project along with a four-fold increase in production has generated new interest in an old asset. This case history reviews how detailed safety hazard analysis and proper planning resulted in a safe and successful operation even under the most extreme design and regulatory requirements.

Introduction

Four Star Oil & Gas, a subsidiary of Texaco, operates the Hatter's Pond field nineteen miles north of Mobile, AL (fig. 1). Discovered in 1974, the field has produced in excess of 210 billion cubic feet of gas and 50 million barrels of condensate and natural gas liquids. This north-south anticline produces from two Upper Jurassic formations. The Smackover, a shallow marine dolomite, occurs at depths ranging from 18,000 to 18,300 ft and is 15 to 150 ft thick throughout the field. The underlying Norphlet, an aeolian sand dune sandstone, is 0 to 300 ft thick. The 100 to 150 ft thick Buckner Salt, overlying the Smackover, along with the eastern boundary (the western fault of the Mobile Graben) form the trapping mechanism that creates the reservoir. Arial extent of the field is 1 mile wide by 5 miles long where reservoir porosity is 12 to 15% and permeability averages 2 to 3 md. With bottom hole temperature at 350° F and hydrogen sulfide concentrations of 100 to 700 ppm, the formations present formidable obstacles for both drilling and production.

The aging depletion drive reservoir has seen average bottom hole pressures (BHP) drop from an initial 9200 psi to the present day 2700 psi. Originally, gas injection wells were used to maintain BHP thereby maximizing condensate recovery. The field has reached the blowdown phase and all producible hydrocarbons are being extracted. The typical well presently produces 3 to 6 MMscf/day and 100 to 400 bbls/day condensate through a 5–1/2" production liner/tieback (fig. 2).

Problem

- Based on reservoir modeling, Texaco's Gulf Coast Asset Team investigated possibilities for sidetracks and new-drill candidates. With stringent economic constraints, the Gulf Coast Drilling Team worked closely with the asset team in preparing cost estimates that might yield consideration. Due to low BHP (2.9 ppg EMW), the drilling of previous wells with conventional drilling fluids experienced considerable fluid losses. Additionally, this fluid invasion resulted in formation damage contributing to low flowrates from completions. Consequently, conventionally drilled wells showed little potential.

Solution

- The drilling team looked for technology that would solve the problem and found that underbalanced drilling (UBD) with foam or gas could mitigate the aforementioned difficulties. The prospect of drilling these formations in an underbalanced state, where reservoir fluids are made to flow into the wellbore, generated much interest. Additionally, the idea of executing an openhole completion to maximize surface area for flow was proposed with high expectations.

However, while in the planning stage, this idea presented an entirely new set of problems. The drilling team had limited experience with UBD and no experience with gas drilling. Also, the Alabama State Oil & Gas Board needed to allow exceptions to State Rules for the project to be implemented. Both of these issues were overcome, and UBD was safely and successfully performed. The results created interest in this mature asset.

Problem

- Based on reservoir modeling, Texaco's Gulf Coast Asset Team investigated possibilities for sidetracks and new-drill candidates. With stringent economic constraints, the Gulf Coast Drilling Team worked closely with the asset team in preparing cost estimates that might yield consideration. Due to low BHP (2.9 ppg EMW), the drilling of previous wells with conventional drilling fluids experienced considerable fluid losses. Additionally, this fluid invasion resulted in formation damage contributing to low flowrates from completions. Consequently, conventionally drilled wells showed little potential.

Solution

- The drilling team looked for technology that would solve the problem and found that underbalanced drilling (UBD) with foam or gas could mitigate the aforementioned difficulties. The prospect of drilling these formations in an underbalanced state, where reservoir fluids are made to flow into the wellbore, generated much interest. Additionally, the idea of executing an openhole completion to maximize surface area for flow was proposed with high expectations.

However, while in the planning stage, this idea presented an entirely new set of problems. The drilling team had limited experience with UBD and no experience with gas drilling. Also, the Alabama State Oil & Gas Board needed to allow exceptions to State Rules for the project to be implemented. Both of these issues were overcome, and UBD was safely and successfully performed. The results created interest in this mature asset.

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