Cathodic Protection of Well Casings Offshore
Authors: Schremp Fred W. 
Published: December 2007

Kickoff operations performed with a mud motor and a bent sub may induce dogleg production when weight on bit (WOB) varies. To combat this, a constant lateral force can be applied immediately above the bit. We have developed and used such a technique successfully in the form of an offset stabilizer since 1977.

Introduction

Our company has been developing Umm Shaif and Zakum fields offshore Abu Dhabi since 1958. Of 279 wells drilled in these fields, 200 are deviated wells.

The kickoff was carried out conventionally by use of a mud motor/turbine with a bent sub-a lengthy. costly, and often tricky operation. Several positive- displacement mud motors and turbines were tried.

With the configuration of mud motor/turbine and bent sub. the buildup rate was a function of drilling parameters applied, particularly the WOB. Because we wanted to avoid excessive dogleg severity, we could not use the maximum efficiency of our most powerful tool the 10 1/4-in. (260.4-mm) turbine. We tried to find a means to eliminate the influence of WOB on the buildup rate. Because we could not reduce the bending momentum by lowering the bent sub, we investigated the possibility of developing a lateral force on the bottom of the turbine, right at the bit. The offset stabilizer was born.

Discussion

Our policy up until 1977 was to kick off at a low depth [about 3,000 ft (915 m)] in a 17 1/2-in. (444.5-mm) hole, since only low-departure wells were being planned. Several downhole motors were evaluated for kicking off in a 17 1/2 -in. 444.5-mm) hole, with the early assemblies incorporating a bent sub and a mud motor or turbine. However, as we investigated revision of the kickoff point, our objective was to develop a technique that would satisfy the following requirements,It would smooth the buildup curve during kickoff to minimize dogleg severity and hole reaming before drilling is resumed.It would minimize kickoff time with maximized rate of penetration (ROP).It would use a standard kickoff assembly that could be used for either a 17 1/2-in. (444.5-mm) hole or a 26-in. (660.4-mm) hole.

Trials to achieve our objective were carried out with different downhole motors; however, the results obtained were unsatisfactory. We finally developed, our current technique, involving a 10 1/4-in. (260.4-mm) turbine with an offset (eccentric) stabilizer.

The evolution of our kickoff operations could be summarized as follows.

Kicking off in a 17 1/2-in. (444.5-mm) hole includes (1) a mud motor or turbine with a bent sub, and (2) a turbine and an offset (eccentric) stabilizer.

Kicking off in a 26-in. (660.4-mm) hole involves (1) kicking off in a 17 1/2 -in. (444.5-mm) hole, then opening the hole to 26 in. (660.4 mm), and (2) use of a turbine with a bent sub/offset stabilizer (Fig. 1). Each method is described in the following.

Kicking off in a 17 1/2-in. (444.5-mm) Hole

All the kickoff operations performed until 1977 were done with a 9 5/8-in. (244.5-mm) mud motor or a 10 1/4-in. (260.4-mm) turbine with a bent sub. The kickoff assembly used included (1) a 17 1/2-in. (444.5-mm) milled tooth bit, (2) a 9 5/8-in. (244.5-mm) mud motor and a 10 1/4-in. (260.4-mm) turbine, (3) a bent sub (2 or 2 1/2 degrees), (4) a 15-in. (381-mm) or 17-in. (431.8-mm) string stabilizer, and (5) 60-ft (18.3-m) nonmagnetic drill collars.

JPT

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