The first string of well casing is usually called a conductor The conductor is the well foundation and its main function is to resist the axial and lateral loads imposed at the wellhead. In stiff clays and sands conductors are generally grouted into pre-drilled oversized holes. However, a technique analogous to wash boring, called Jetting, is a feasible alternative for installing conductors in soft clays that are often encountered in deep water.

Jetting can reduce rig operating-time and save about $0.5million per well in deepwater exploration The technique is also inherently safer than open-hole drilling and cementing in very soft sediments but it can be counter-productive because of poor installation practices. The most common operational problem with jetted conductors is excessive settlement under the weight of the next casing string Failures of this type, which arise from insufficient short term axial capacity of the jetted conductor, are episodic rather than frequent but each incident can cost $2million or more if the well has to be respudded. Jetting is used routinely in the Gulf of Mexico (GoM) but drilling and cementing has remained popular in other regions. In the past 3 years BP has promoted the use of jetted conductors m deep water in and outside GoM and has carried out studies to reduce the incidences of axial being capacity failures arising from this initiative.

This paper reviews the jetting technique, summarizes BP Exploration's method for estimating the axial capacities of jetted conductors m soft clays and describes the successful adaptation of the method for deepwater exploration wells m Angola.

Deep-water Wells

Deep-water wells are drilled from anchored or dynamically positioned semi-submersible drill rigs or drill ships. They can be broadly categorised as Exploration Wells or Development Wells. Exploration wells are drilled during the prospecting stage of a potential field. These include wells to prove the presence of hydrocarbons, often referred to as ‘Wildcats’, and Appraisal Wells used to define the size, quality and extent of a discovery Development wells include production wells to recover hydrocarbons, and gas and water injection wells used to enhance recovery and/or dispose of fluid bi-products (produced fluids).

Both types of wells are lined by a series of concentric steel tubes called casing strings that are cemented in place to stabilise the bores and reduce the risk of fracture of the formation while drilling with weighted mud. Unless adapted for production, exploration wells have relatively short design lives. They are also generally planned with &ma1 knowledge of geology, formation pore pressures, and service life loads. Development-wells are drilled later so are usually planned with better information on the subsurface conditions. The design loads for development wells are also better defined than for exploration wells. In each case the main objective of well design is to achieve the total depth safely with the most cost-effective numbers and sizes of casings. An example deep-water exploration well casing design used by BP in Angola is shown on Figure 1.

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