Modern extended reach, horizontal and multilateral wells are often drilled out of closely placed template slots. This situation has resulted in large numbers of wells drilled in close proximity in order to access oil and gas reserves and maximise production. An example of where this need is particularly evident is for the artificial islands project in the Upper Zakum field. With this type of scenario in mind, the demands for precision well positioning have become ever greater in order to enhance production through improved reservoir drainage, as well as increase safety through minimising the likelihood of collision with a producing well and so reduce the risk of a blowout leading to lost revenue and, most significantly, injury to operating personnel and loss of life.

The value in achieving the best possible trajectory information therefore becomes very clear. This paper examines two possible approaches to well positioning; the conventional approach based on magnetic MWD including in-field referencing aiding versus the relatively new all-attitude gyro. The paper highlights the key features of the newer all-attitude gyro technology which is described in SPE paper SPE-168052. Both methods are capable of providing survey data at any attitude, but each is subject to performance limitations resulting from a variety of causes.

Wellbore position uncertainty estimation (error modelling) is a key element in safe and cost effective drilling. The understanding of error modelling has improved greatly over the last 20 years largely through the publication of SPE papers like SPE-67616, 90408, 103734, 105558 and 140192. The principle sources of error associated with both the MWD and gyro survey systems are described in this paper; the resulting propagation of errors and the final positional uncertainty for each case is compared.

Some example case studies, relating to wells drilled in different parts of the world, are presented which illustrate the relative performance of the two approaches to surveying wells. The increase in well positioning accuracy achievable using the all-attitude gyro over MWD is illustrated, an improvement that is particularly evident when drilling high angle sections close to an easterly or westerly direction. A factor of two or more enhancement in accuracy is offered by the application of gyro technology in such cases.

Where well positioning accuracy is so vital to the financial success of an oil field and the safety of operating staff, this capability should not be ignored.

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