Introduction

Starting in late 1997 a major development drilling campaign was undertaken by Chevron in the Carter Creek Field in Southwest Wyoming.The sour gas reservoirs at +/-15,000' TVD were targeted to increase production and improve ultimate recovery in this mature area.Between 1997 and 2002 15 new wells were drilled and completed as a part of this effort at approximately $8 million per well.The formations in this part of the Rocky Mountains are characterized by complex geology, hard abrasive formations, plastic salt, and high concentrations of hydrogen sulfide gas (16%).Additionally, the field is located on environmentally sensitive federal lands.Directional drilling is generally necessary to reach the desired bottom hole locations from the available surface access routes.

Overall, the program was very successful, resulting in 5–8 BCF of incremental gas reserves per well.However, by late 2002, bottom hole pressure had declined to the point that the new wells were becoming marginally economic.Although a core area and major capital project for the operator, it became clear that the remaining reserves could no longer support the associated well construction cost.The viability of a large gas plant and more than 100 BCFG recoverable reserves were at stake.

During 2002 and early 2003 multi-discipline team work was undertaken to assess possible alternative well designs utilizing a variety of risk analysis tools.The result was a slim hole well design enabling the use of a smaller drilling rig and an overall savings which to date has approached $1 million per well.This paper details the evolution of a new well design which balances geological and production objectives with the increased risk related to a smaller hole size.At present, the new well design is being utilized at increasing depths with continued efficiency gains.The associated well construction cost savings have become instrumental in maximizing value from this important asset.

Background

The Carter Creek Field is located in the Rocky Mountain Thrust Belt +/-25 miles north from Evanston, Wyoming and was discovered by Chevron in 1977.The Mission Canyon gas reservoir in this field is located @ 15,000' TVD as a part of a complex system of thrust belt geology.Extremely hard, abrasive formations, plastic salt and steeply dipping beds are characteristic of the area.Surface topography is characterized by remote, 7,500'+ elevation, mountainous terrain and high winds where sub-zero temperatures are common during long winter months.

These site-specific characteristics have presented many challenges to operators in this area over the years.Prior to 1997, exploration and development drilling to the Mission Canyon in this area required 150–200 days to construct large mountain-side locations to accommodate 3000+ HP drilling rigs with large reserve pits, and directional drilling operations to +/-17,000' MD.The Federal permit process generally precedes spud by 6 months or more, and completion operations with high rate acid fracture treatments are significant projects in themselves.Location construction and rig moves are scheduled during favorable weather windows if possible.

Prior to 1997, the significant cost, geologic, and economic risk would often result in one or two well campaigns where learning curve benefits were difficult to achieve.Business unit managers often wanted to see results from the first well before approving subsequent wells.Permitting time and weather windows would then necessitate that the drilling rig be released and re-started when the next location was ready.

Another significant issue at Carter Creek is the competitive lease environment.During the early stages of field development in the '80's offset operators failed to come to terms and form a field wide unit.This eventually resulted in a ‘checker board’ lease situation.Two separate gas plants were constructed with competitive production / reservoir drainage set in motion.

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