Drilling Fluid Design to Prevent Formation Damage in High Permeability Quartz Arenite Sandstones

Abstract

Quartz arenites are very clean quartz sandstones with unusually high permeabilities for any given porosity. This is attributed to the presence of large, clean pores which are often well connected. These large pores are susceptible to drilling damage, mainly from the invasion of solids. This paper presents both laboratory and field data on the development of drilling fluids to minimise damage in quartz arenites. The required changes in field mud engineering practices, to achieve improved well performance, are discussed. Field experiences are presented, including the impact of the modified mud systems and engineering practices on the resulting skin factors. Introduction Quartz arenites are very clean quartz sandstones with unusual poro-perm properties. Significant permeability is retained at low porosities and as the porosity increases the observed permeabilities are much higher than would normally be predicted. This unusually high permeability is attributed to the presence of large, clean pores which are often well connected. Despite these favourable characteristics, many wells drilled in quartz arenite reservoirs have not been as productive as expected, and have had high skin factors. In the absence of reactive clays, many recognised damage mechanisms are not applicable in quartz arenites, yet drilling damage is still seen to occur. Laboratory data shows that conventionally designed drilling fluids can cause high levels of damage, but that mud filtrates alone do not cause significant damage. The main damage mechanism is solids invasion into the large pores, and it is shown that changes in drilling fluid design can minimise this damage. These modifications to the drilling fluid require changes in field mud engineering practices, in order to achieve improved well performance. Field experiences are presented, including the impact of the modified mud systems and engineering practices on the resulting skin factors. The Mirador Formation Quartz arenites are clean quartz sandstones, which contain more than 95% quartz. The Mirador formation in the Cusiana field in Colombia is a quartz arenite, with a very low clay content. The typical mineralogy is 78% quartz grains, 14% quartz cement and 8% porosity. Despite low porosity, permeabilities are high. For example, 8% porosity corresponds to about 100mD and 10% porosity to about 4o0mD, although permeability is dramatically affected by grain size. The permeability of the very coarse grained sand is about 800mD at 10% porosity, whereas the permeability of the fine grained sand is about 90mD at the same porosity. The completed intervals have a wide range of permeability, sometimes from less than 10mD to more than 5000mD (the >1000mD sands generally have low thickness). The Mirador has very low vertical permeability, compared to horizontal (Kv/Kh = 0.1 or less, dependent on formation permeability). Reservoir pressure is approximately 5300psi and reservoir temperature is 127 F. Pore Size Distribution. The high permeability at low porosities is a result of large pore size and good interconnectivity. The median (D50) pore size in the lower Mirador is usually about 60, although in the high permeability streaks the D50 pore size has been measured as high as 110. More importantly, typical pore size distributions show the D95 to be substantially higher than the Dso (Fig. 1). As an example, a core with Dso pore size of 60 will have a D95 of 350-400. By converting a typical pore size distribution to a predicted permeability distribution (Fig. 2) it is apparent that the large pores dominate the permeability. In the example presented, the largest 30% of the pores account for about 85% of the permeability. Consequently, the large pores need to be protected in order to minimise damage. Compressive Strength. As a result of the secondary quartz cementation, which produced the low porosities, the Mirador sands also have high unconfined compressive strengths. P. 147

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