An Experimental Investigation of Permeability and Porosity Alteration in Diatomite During Hot Fluid Injection

Abstract

Abstract A study of silica dissolution has been performed to probe the evolution of permeability and porosity in siliceous diatomite during hot fluid injection, such as water or steam flooding. Two competing mechanisms were identified. Silica is soluble in water at elevated temperature causing rock dissolution and thereby increasing permeability; however, the rock is significantly compressible leading to compaction of the solid matrix during injection and the loss of permeability and porosity. A laboratory flow apparatus was designed and built to examine these processes in diatomite core samples. At the core level, we measured the pressure drop as a function of time to determine the permeability variation and utilized an X-ray Computerized Tomography (CT) scanner to measure porosity. At the pore level, a scanning electron microscope (SEM) was used to observe changes in pore morphology. We found that porosity decreased initially due to compaction caused by the imposed pressure drop across the core. Later, porosity increased as silica dissolved. Dissolution of the rock matrix was relatively uniform in that wormholes were not observed even after tens of pore volumes of fluid injection. Introduction Diatomaceous rock is composed of biogenic silica, detritus, and shale in different proportions depending on its origin. The characteristics of this rock are high porosity (25-65%), rich in oil (35-70%), but low permeability (0.1 to 10 md) [1]. Cumulatively, diatomaceous petroleum reservoirs in the San Joaquin Valley, California contain roughly 12 billion bbl of original oil in place [2]. Because the permeability of diatomite is very low, compared to typical sandstone reservoirs, the recovery of oil by usual techniques is difficult. Steam drive as a means to recover heavy and medium oil from diatomite has been tested in the South Belridge and Cymric fields (Kern Co., CA) and been found to be technically successful [3-6]. Steam injection, however, may lead to new complications because of rock dissolution and precipitation and the ensuing evolution of pore morphology [7,8]. Additionally, relatively high injection pressures are necessary to force steam to enter a low-permeability rock matrix. This may lead to fracturing of the rock matrix in extreme cases. An isothermal laboratory apparatus was constructed and one-dimensional fluid flow studied at a variety of flow rates and elevated temperatures to unravel these competing effects. The effect of hot, fresh water is our focus because it is most relevant to steam condensation and resulting water flow near injection wells. Computerized tomography (CT) and scanning electron microscope (SEM) images are used to monitor experimental progress in addition to conventional measurement of permeability. It is difficult to conduct laboratory experiments precisely at reservoir conditions. Thus, we performed a scaling analysis to differentiate laboratory and field conditions. The important dimensionless parameters are the Peclet, Pe, and Damkohler, Da, numbers [9]. The Peclet number is the ratio of the characteristic time for diffusion upon time for convection, while the Damkohler number is the ratio of the characteristic time for fluid convection upon the time for dissolution. The product of Pe and Da numbers is the important scaling parameter and if this product, PeDa, is less than 1 the process is reaction limited and dissolution is nearly uniform. On the other hand, the process is transport limited for PeDa greater than 1. Selective dissolution occurs along the most permeable flow paths and permebility evolves nonuniformly (PeDa>1).