atocha project, page-7

Heres some more info

Drilling the Tuscaloosa formation in Louisiana was as challenging as any area drillers worked 20 years ago. It was one of the most active areas in the U.S. Problems that were new at the time such as gas solubility in oil base mud, sulfide stress cracking of tubulars, pressure regressions and low margins between the fracture pressure and pore pressures resulted in numerous well control problems and high cost wells. Activity decreased in the mid 80's when the gas price did not encourage operators to challenge the drilling risks. BP Amoco is still successfully drilling to 22,000' into to 16,000 psi, 380°F reservoirs that are now depleted over 10,000 psi.

Introduction

The Tuscaloosa Trend is a band of Upper Cretaceous sand 30 miles wide and 275 miles long. It extends from the Louisiana-Mississippi border through Baton Rouge to the Texas border. The center and most productive portion of the field is located north west of Baton Rouge. Drilling began for the Tuscaloosa formation by a Chevron discovery in 1975. A blowout of the Chevron Parlange #1 was controlled to sales at a rate of 140 MMCFD1 for two months in 1977. With that demonstration of the formations' capacity the boom was on. By September, 1980, there had been 70 wells drilled2, with 16 productive. Seventy one wells were drilling at that time.

Activity peaked in 1981 when 80 rigs were drilling for 20 operators.

The early drilling had many problems. Problems were associated with the high activity, lack of understanding of the Trend, and not recognizing some technical problems.

Oil base mud was widely used due to the high temperature, deep depths, corrosive gases, shale problems and small holes at TD. The solubility of gas, particularly methane, in oil base mud was not well understood in the early drilling3. Gas went into solution when formations were drilled. This gas was then circulated to the surface. When the hydrostatic pressure had been reduced on the entrained gas by the circulation, the gas then broke out of solution often pushing the top few thousand feet of mud column out of the well. The large gas volume presented an immediate danger to the rig and crew and the reduction of hydrostatic created a continuing well control problem4.

Early 80's was a boom time in the oil field. The number of land rigs working domestically was over 4100 in 1981, compared to less than 600 today. The high rig demand spread limited experience over a large number of rigs. The demand also created crew retention problems and brought unsatisfactory equipment into play. The supply of rig sight supervisors was stressed to the point that many consultants could work for months without relief. The incomes of consultant “company men” would benefit at the expense of quality supervision.

Tubular supplies for the large number of rigs was also straining the system. Competition for acreage resulted in leases with drilling demands. Thus, rig demand was high. That demand pushed the need for tubular goods. The deep, high pressure, sour gas reservoirs were creating demand for tubulars that had not been commonly used in the oil field. The unique tubulars required more lead time.

BOPs, choke manifolds and drill pipe were also a stress point that the high demand aggravated.