Abstract

Following encouraging results of Under-Balanced Drilling ("UBD") applications in the North Sea and onshore Canada, Shell Malaysia Exploration and Production ("SM-EP") embarked upon the application of this technology in a shallow clastic oil field in offshore Sabah. Relatively little information is available in the public domain regarding the effectiveness of UBD in clastic oil reservoirs. This paper provides factual evidence relevant to the assessment of UBD-induced added value. The paper also provides a comparison between the cost and benefit of UBD with that of Under-Balanced Perforating ("UBP") in a two-well trial.

Every pressure regime, reservoir fluid type, geological circumstance, etc, requires a specific operational configuration custom made for the situation at hand. This is a major source of UBD-induced incremental cost. Different reservoirs require different UBD configurations so analogies that assist the business case of a UBD project are currently limited and are difficult to find. This was the first offshore application of crude oil and hydrocarbon gas injection as drilling fluid in jointed pipe operations. A description of the incentives and risks of our specific geological circumstances is provided. Detail will be provided regarding our UBD design, main operational-risk mitigation measures, and the strategy used to appraise the added value of UBD. The complexities, incentives, and pitfalls of our UBD strategy will be chronicled. The initial results of this first-ever UBD trial within SM EP will be published, along with the lessons that we have learned.

Introduction
Field Background

The St. Joseph ("SJ") Field (figure 1) is a Miocene aged sequence of "layer-cake" sandstones and shales deposited in a storm/wave influenced shallow marine environment. This highly stratified sequence is some 1500-ft thick, sharing a common initial pressure regime and aquifer. The oil accumulation resides within a dipping flank (~ 20 deg) of an anticlinal structure. The accumulation is at a mature stage of development, currently producing from 65 production strings. The reservoir pressure has been depleted to approximately half the initial pressure. Natural pressure support is weak, hence artificial pressure maintenance, in the form of crestal gas injection, has been in place since 1996. This together with subsequent downdip infill drilling has arrested decline in the field's production rate. The producing sections of these downdip infill wells comprise of long horizontal sections (1500–2500 ftah) that intersect all of the dipping layered stratigraphy. These are completed with cemented liners and are perforated for co-mingled production. The UBD wells mentioned in this paper have a similar configuration with the exception that the toe sections of the wells are UBD. These toe sections are completed with a 4.5-inch pre-drilled and uncemented liner (figure 2).

UBD Theory

It is widely acknowledged that a consequence of conventional over-balanced drilling is well-bore formation damage. The overbalance caused by the hydrostatic pressure of the drilling fluid column (in order to contain the reservoir pressure) forces the intrusion of foreign fluids and particles into the originally pristine pores and pore-throats of the invaded zone. Chemical changes of clays, in addition to the intrusion of fine particles cause restrictions within the pore-throats leading to a reduction in permeability. Formation damage in sandstone reservoirs is generally difficult to remove, in contrast to carbonates that usually respond much better to acid stimulation. UBD is a drilling technique that can potentially avoid this damage to sandstone by the avoidance of an invaded zone altogether.

The technique applies lower weight drilling fluids to reduce bottom hole pressure ("BHP"). When the BHP is lower than the reservoir pressure, flow into the wellbore occurs. This effectively avoids the creation of an invaded zone that is damaged. Sensitive clay structures remain intact, and all pore-conduits should remain free of debris/blockages as in their original pristine state. This outcome potentially realizes an idealized situation where only a Darcy associated pressure drop occurs when oil flows from the reservoir into the wellbore (i.e. zero skin).

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