Two exploratory wells were recently drilled in the North Sea which used a specially designed polymer drilling fluid to provide optimum shale and borehole stabilization. The first well was a deep Jurassic well in the Frigg and Balder area of the Norwegian North Sea; the second well was drilled in the Ekofisk area. (Figure 1.)
FIGURE 1

SHALE STABILITY MECHANISM

FIGURE 1

SHALE STABILITY MECHANISM

The mud for the reactive formation intervals was formulated to provide chemical stability for the reactive tertiary clays and claystones. Conventional fluid loss and rheology control polymers (polyanionic cellulose and xanthan gum, respectively) were used to achieve and maintain the required performance properties. Shale stability was obtained using a novel.additive composed of a polyvinyl alcohol polymer, potassium silicate, and potassium carbonate.

Shale stabilization is based on a unique new theoretical mechanism which was initially postulated in SPE 16687, "New Chemical Package Produces an Improved Shale Inhibitive Water-Based Drilling Fluid System," and presented at the 62th SPE Conference, Dallas, Texas, 1987 (Wingrave et al). Further details are included in this paper.

In the deeper intervals, high temperature polymer additives were used to maintain favorable rheological and filtration properties.

An injection system was used to inject prehydrated liquid polymer concentrate directly into the suction. This procedure was used to obtain optimum polymer efficiency and performance in the management of the drilling fluid system.

Drilling and drilling fluid performance data is presented for the 17.5", 12.25", and 8.5" hole sections. Mud weights from 10.0 to 16.0 ppg were required in the drilling program.

Drilling and fluid performance data from the first well (Frigg and Balder area), designated Well A, is compared with an earlier offset well in the same block, Well B, which was drilled with a conventional KCl/PHPA/Biopolymer/PAC polymer system.

Caliper data, borehole stability relative to exposure period, solids removal efficiency performance, wireline log quality, trouble-free casing and cementing operations, and minimal fluid-related hole problems while drilling - all provide supportive data indicative of superior chemical stabilization of the wellbore.

U.S. and Norway toxicity results support its use as an environmentally acceptable water base additive for offshore and onshore application.

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