How Using Oil Mud Solved North Sea Drilling Problems

Abstract

Oil mud has been used to solve severe problems encountered while drilling from a 40-well platform in the North Sea. Problems were eliminated or greatly reduced in severity, and costs were less than when using the best water-base muds. A new one-package oil mud and cuttings processing system was introduced, and total responsibility for the mud system was given to a mud service company. Introduction Need for Special Drilling Fluids During the early exploratory work in U.K. Block 9/13 and Norway Blocks 33/9 and 33/12, drilling problems consumed an unsatisfactory amount of problems consumed an unsatisfactory amount of time. When commercially important oil fields were discovered within these blocks, the operator asked for a review of the drilling operations with special attention to be given to the effects of various drilling fluids on wellbore conditions. The objective was to select the best fluids for continuing the exploratory work and development of the fields.The drilling review took place in Sept. 1974. Examination of the drilling records available at that time produced the lithological description shown in Fig. 1. This was to have a very important part in the selection of the drilling fluids. Siltstones, claystones, unconsolidated sands, marls, and shales were the principal formation materials. Some coal seams also principal formation materials. Some coal seams also were found later to be part of the lithology. Most of these materials were found to react almost identically with any type of water-base mud. As drilling progressed, lime, seawater, various lignosulfonate progressed, lime, seawater, various lignosulfonate muds, and the rather new potassium chloride systems were tried. Caliper logs of the wells showed hole enlargement in the same formation, almost always of about the same degree. A lesser amount was observed with the potassium chloride system; however, this was attributed not only to the inhibitive qualities of this drilling fluid but also to the control exercised over the viscometric properties to keep the annulus flow in the laminar state. The operator became convinced of this through comparison of wells drilled by this fluid but having different viscometric properties and annulus flow rates. Hole erosion and properties and annulus flow rates. Hole erosion and cleanout became factors of chief concern. It was easy to show that hole enlargement was the source of many of the hole problems, stuck pipe, reaming through bridges and hard streaks, and the failure to be able to set adequate cement plugs. The consumption of cement during casing operations was excessive. For example, in one section the hole was enlarged so much that the casing job that ordinarily would have cost $7,000 for cement (assuming gauge hole) actually cost $29,000.Upon completion of the drilling review, the following conclusions were presented to the operating management.1. If water-base muds are used to drill the wells from the development platform, hole enlargement may be so serious that hole and casing interference problems could arise. These problems not only could problems could arise. These problems not only could be costly to handle, but they also could cause a reduction in the number of wells that may be drilled from the platform. (Forty wells were being planned.) JPT P. 931