Studies on the Damage Induced by Drilling Fluids in Limestone Cores

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractWell productivity can be seriously reduced by excessive fluid loss and solids invasion while drilling. If not adequately controlled, losses may push hydrocarbons very far from the hole and solids may plug the pores, increasing the time necessary for testing the well and distorting the evaluation of its productivity potential.Laboratory procedures for determining the damage of a mud system on rock samples are traditionally based on the return permeability test which provides an indication of both the damaging potential of mud and the efficiency of cleanup techniques.The non-destructive nuclear magnetic resonance (NMR) measurement is a new methodology for studying the influence of small scale geological heterogeneities on fluid flow and trapping. It permits to check the invasion of solids and filtrate present in the mud using different experimental set-ups. The main advantage is the possibility to have the spatial distribution of the invading phase inside the rock sample. Low frequency (2 MHz) nuclear magnetic resonance relaxation (NMRR) measurements were also used for following the effect of solid mud particulate invasion on the pore size distribution. These techniques provide fast and reliable analysis without the destruction of the sample. This paper reports the laboratory procedures and the results of a study aimed at determining the damage coming from the penetration of bentonite/mixed metal particle aggregates and polymers into low permeability carbonatic reservoirs. The results were compared with both traditional and innovative methods. Combined with spurt loss, non conventional rheological approaches, dynamic filtration and mud cake SEM (scanning electron microscopy) observations, nuclear magnetic resonance imaging (NMRI) provides precise and directly observable results to distinguish the severity of damage, the type and depth of invasion and, even more, the distribution of invaded particles in the matrix.The integration of conventional and advanced methods to study damage appears to be highly promising for selecting drilling fluids, with suitable components and particle size distribution appropriate to protect the reservoirs.