Oil Mud Aids in Reducing Problems and Cost of North Sea Platform Development Drilling
Authors: Holder Barry John 
Published: December 2007

Holder, Barry John,* SPE, NL Petroleum Services (U.K.) Ltd.

Summary
The benefits of using oil muds on a development platform in the North Sea are discussed together with special equipment necessary to minimize risks of oil pollution from the circulating mud system during drilling. A cost comparison of using oil mud vs. water is shown.

Introduction
The Montrose platform in Block 22/17 in the U.K. sector of the North Sea is an outstanding example of problem solving and cost saving through application of good drilling practices and use of oil mud. The principle and application of oil mud to this drilling project are discussed in this paper and supported by field results.

History of Montrose Field Drilling
Initially this field proved a very troublesome drilling area when drilled with water-base mud. The first discovery wells in this field were drilled in 1972 with the Sea Quest by using a KCl/starch mud system. The vertical wells drilled to evaluate the field did not prove too troublesome, but when the Montrose platform was erected and the first development well was drilled, the wells had to be plugged and abandoned at a depth of 4,765 ft after the hole caved in. The next six wells were completed successfully with water-base mud, but poor hole stability caused four plugback and sidetrack operations. An average of 43 days was required to drill these wells to total depth (TD), 1,000-ft average, at an average maximum angle of 48 degrees for an average total mud cost of approximately $350,000. Compare this performance with that for the 11 wells drilled with oil mud. An average of 16 drilling days is required to drill from 18 5/8-in. casing point to 11,800-ft average TD at an average maximum angle of 53 degrees for an average mud cost of less than $200,000. Details are given in Table 1.

Oil Mud vs. Water Mud Chemistry of Oil Mud
Why is a hole drilled with oil mud more stable than one drilled with water mud? The main reasons are that with an oil mud (1) the continuous phase of the mud is diesel oil and (2) clays, which can be up to 90% or more of the formations down to the pay zone in the North Sea area, do not yield in an oil environment. This eliminates the need for high dilution and resultant mud treatment to maintain (1) low solids content in the mud and (2) control of the mud properties. Borehole walls do not hydrate or slough, and remain stable until the section is cased off.Once a hole is drilled through a clay formation with a water-base mud, the clay acts like a sponge and tries to imbibe water from the mud with the same force that squeezed the water of the sediment when it was laid downi.e., a force equal to the matrix stress of the rock caused by the overburden weight of the rocks above the claystone. That is why claystone cuttings in a waterbase mud arrive at the flowline as "gumbo." They are in a partially hydrated state. Actually the claystone is in situ as a relatively hard and dry rock. Various water-base drilling fluid additives slow down the rate at which the claystone imbibes water.These additives include soluble electrolytes such as potassium chloride, sodium chloride or lime, and polymers that coat the claystone. With any water-base mud, however, the clay eventually takes on water, resulting in sloughing of the borewalls and/or dispersion of the clay cutting into the aqueous environment as it travels up the annulus. The rate at which this occurs can be slowed down by changing the chemistry of the mud, and the rate depends on the degree of inhibition of the mud and the reactivity of the clays. An important feature of an oil-base mud is that it contains a percentage of water, emulsified into the continuous oil phase to form an invert emulsion.
JPT
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