Fracture Width LWD and Drilling Mud / LCM Selection Guidelines in Naturally Fractured Reservoirs
Authors: Lietard Olivier , Unwin Tessa , Guillot Dominique , Hodder Mike 
Published: December 2007

This paper discusses drilling mud losses in naturally fractured reservoirs with fracture permeability larger than fifty odd millidarcies (and generally much larger than the matrix permeability).

Type curves are provided that describe mud loss volume vs. time and allow for the determination of the hydraulic width of natural fissures through the usual type curve matching technique of log/log plots.

The method also gives access to the number of fissures intercepted by the drilling bit at the onset of the loss.

Real time "LWD" of natural fracture width permits the proper selection of LCM with respect to particle size distribution. Efficient (rapid) mud loss control is made possible, increasing the chances at minimising fissure damage by the drilling mud, the cuttings and the LCM themselves.

Total mud losses experienced when drilling a long horizontal section are shown to agree with reservoir properties (derived by well testing) in particularly well-documented examples of the Central North Sea.

The provided equations can therefore be utilised as well by reservoir engineers to properly evaluate the contribution of natural fissures to the permeability and skin of any well, these fractures being clean or invaded by drilling mud.

Introduction

The idea of monitoring massive mud losses in a naturally fractured reservoir to characterise formation rock porosity's was proposed about thirty years ago and is therefore nothing like a new one. Despite a sizeable list of studies since, however, only qualitative methods have been derived so far, as evidenced by the most recent publications.

The increasing occurrence of oil field developments in fractured reservoirs in the North Sea has led lately to even more dedicated efforts at understanding the impact of large mud losses on formation damage and damage removal techniques. However, the need to come tip with a detailed quantitative analysis of the phenomenon has become urgent with the perspective of the start-tip of the Clair field west of Shetlands. This huge oil accumulation is the first and so far unique example of naturally heavily fractured sandstone reservoir in this part of the world. Its successful development cannot be guaranteed unless fissure damage by insoluble cuttings is minimised, if not prevented. Indeed, the large viscosity of the reservoir oil at downhole conditions would not allow for economical production if skin was present. Methods to ensure immediate control of mud losses must be designed and implemented. They cannot be based on anything else than a precise, real time knowledge of the width of the fissure the drilling bit is running through.

In addition, the determination of the actual hydraulic width of natural fractures when drilling wildcats and appraisal wells would make extended well tests (EWT) much less necessary. Formation microimaging (FMI and other related acronyms) indeed has only access to visual width and is merely reliable when extremely near-wellbore values of fissure width and spacing are required. It is the industry consensus that the data derived from such logging tools must not be extrapolated at the scale of the reservoir. Hence the requirement for a kind of logging while drilling (LWD) technique with a meaningful investigation radius.

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