Slim-hole Drilling on the Gulf Coast

Abstract

The cost of drilling in the past few years of proration and ever decreasingallowables has received increasing thought and study. It seems to parallel thestrides made with respect to pumping problems in 1932 and 1933. Low profitmargins stimulate the search for economies. Any discussion of slim-hole drilling involves a multitude of abstract values, and the problem can be attacked only by a study of records and rationalthinking. In a measure, traditional methods must be studied with a questioningattitude. The words "slim hole" may be somewhat misleading, as the term isentirely relative. For that reason I wish to define it as used in thisdiscussion. On the Gulf Coast, I would define a normal hole as one in which 103/4-in. surface casing has been set and below which a 9 7/8-in. hole is dug.With this as a datum, holes of larger diameter below surface would be called"large holes," with 12 1/4 in. as the "extra large" class andholes of smaller diameter classed in the slim-hole range, 6 1/4-in.representing about the minimum size. The optimum size has not been definitelydetermined, but the preference is for a program that would call for setting 85/8-in. surface casing and digging a 7 3/4-in. hole. Different sections of thecountry might very properly deviate from this practice because of differencesin depth, formations, regulations, or other factors. This paper will be confined to a discussion of the drilling of exploratorywells, as the drilling of this class of wells is quite a different problem fromthat of drilling a proven field, although many of the same considerations applyin both cases, as will be apparent. Although for many years shallow slim-hole and core-test holes of small diameterhad been dug, it was not, I believe, until the fall of 1937 that seriousconsideration was given to its application in drilling wells 7000 ft., 8000 ft.and 9000 ft. deep on the Gulf Coast. This does not mean that nearly all suchwells drilled on the Gulf Coast now are of the slim-hole variety, but only thatefforts have been aimed in that direction with very encouraging results andthere has been a very definite trend to reduce the size of holes, a trend thatprobably will continue. As to exploration policy, there appear to be two distinct and opposite views.The one usually followed in the past was more or less designed to "shootthe works" on one well. Generally on the Gulf Coast, the depth at whichproduction might be obtained is limited by the drilling equipment andeconomics. With this thought in mind, wildcat wells are often planned with theidea of carrying the depth to 10,000 ft., or even 15,000 ft. All of thepossible hazards are anticipated and the general drilling and casing programsare so predicated. Such wells contemplate the drilling of large-diameter holes;the setting of at least one protective string of casing and an extensive coringpolicy, all of which finally adds up to an expensive well. T.P. 1305