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Abstract

Historically, application of open hole gravel packs have mainly been limited to completion of deviated and vertical wells. Other limiting factors affecting selection of this type of completion are reservoir quality and heterogeneity, or the need for selective completion. However, with an increasing number of high angle and horizontal wells being drilled, the industry is focusing more attention on how to use gravel packs to greater effect in long heterogeneous sections. Keys to success are productivity, management of formation damage through optimal filter cake design, and attainment of long term sand control by ensuring complete gravel placement over the entire producing interval.

This case history discusses a high angle open hole gravel pack application for a sand shale sequence. Inhibition of these highly reactive intra reservoir clays was achieved using a low solids formate based mud system. Full gravel placement along the high angle (75°) reservoir section was accomplished using an alternative path gravel pack technique. Filter cake removal, unlike other industry approaches using acids, relied on application of enzymes to enhance the clean-up/back production of filter cake through the oversized gravel and so maximise productivity.

The discussion will focus on the system design approach to drill-in fluid design / clean-up, and the actual gravel pack placement. Finally, the productivity performance, as deduced from pressure transient analysis, will be presented to demonstrate the success of the unique clean-up approach adopted for this operation.

Introduction

An integrated, holistic approach was taken during the planning, installation and production phase of this well completion. Good communication of the key issues between the various technical disciplines was regarded as critical to ensuring a robust life of well completion, consistent with the reservoir management strategy, facilities constraints and maximising rate of return, whilst also minimising any environmental or HSE impact.

In addition, assessment and management of the productivity and sand control integrity risks (Figure 1.0) helped ensure a trouble free operation.

Reservoir

The reservoir comprises sand/shale sequences, with a net-to-gross of approximately 60%. The intra-reservoir shale comprises layers varying in thickness from several meters to a less than a millimeter, and discrete clasts or inclusions within the sandstone bodies.

It also contains a high levels of reactive smectite clay. The individual sand bodies comprise clean, well sorted, 3-4 Darcy unconsolidated sands, of D₅₀ 250μm and UC 2.0 (based upon dry sieve analysis). However, when pay and non-pay are combined, the formation has a significant fines tail.