Do Wormholes Play A Role In Heavy Oil Cold Production?

The simultaneous extraction of oil and sand during the cold production of heavy oil generates high porosity channels termed “wormholes”. Wormholes grow in a 3D radial pattern within a certain layer of net pay zones, resulting in the development of a high permeability network in the reservoir, boosting oil recovery. Meanwhile, the development of wormholes causes reservoir pressure to fall below the bubble point, resulting in dissolved-gas coming out of solution to form foamy oil. Amplitude anomalies in the vicinity of the borehole observed in time-lapse seismic survey are possibly a result of the presence of foamy oil and wormholes. The foamy oil effects on seismic responses have been discussed at the CSEG convention of 2003. Here, we address the use of time-lapse reflection seismology to detect the effects of wormholes. Tremblay et al. (1999) observed in the laboratory experiments that the diameters of wormholes are relatively small, on the order of centimetres, but the porosity of the mature wormholes could reach 50% or more. With porosities greater than the critical porosity within wormholes, both frame and shear moduli of the suspension sands could significantly decrease. The shear modulus tends to zero in the suspension sands. This leads to the decrease of both P-wave and S-wave velocities of the wormhole sands. Althrough individual wormholes cannot be imaged as their diameters are of the order of centimeters, macroscopic effects of wormholes on Pand S-wave velocities of the reservoir rock can be possibly observed, when large amount of sands are produced, and high dense wormholes exist. 2D numerical models have been designed to examine the footprints of wormholes from both PP and PS seismic data. The modeling results indicate that PS data can better image the wormhole footprints than PP data. The increase of amplitude anomalies appears to be proportional to wormhole density. Introduction Cold production of heavy oil is a non-thermal process, in which sand and oil are produced simultaneously. This process has been economically successful in several heavy oil fields in Alberta and Saskatchewan. The extraction of sand creates a wormhole network and a foamy oil drive. These two effects are thought to be the main influences in enhanced oil recovery. The foamy oil effects on seismic responses during heavy oil cold production surveys have been discussed at the 2003 CSEG convention (Chen et al). This paper will focus on the wormhole effects on seismic data. The cold production process is a pressure driven process, in which both heavy oil and sand are transported to the surface using a progressive cavity pump. This extraction causes high pressure gradients in the reservoir resulting in the failure of the unconsolidated sand matrix. The failed sand flows to the well, triggering the development of wormholes. Sawatzky et al, (2002) believe that wormholes grow in unconsolidated, clean sand layers within the net pay zone, along the highest pressure gradient between the borehole and the tip of the wormhole. These grow in a radial pattern to a distance of 150 m or more from the wells, forming approximately horizontal high permeability, porosity channels in the reservoirs. Laboratory experiments conducted by Alberta Research Council (Tremblay, et al 1999) indicated that the porosity of mature wormholes could reach 50%, and the wormhole diameters could range from the order of 10 cm to one meter (Figure 1). With porosities greater than the critical porosity, the sands within the wormholes are in a state of suspension. Figure 2 shows a critical porosity φc of sandstones. The critical porosity is defined as the porosity that separates their mechanical and acoustic behaviour into two distinct domains (Nur, et al 1998). In other words, the critical porosity can be thought of as a critical value to separate the compressibility and rigidity of saturated rocks into two distinct domains, which are frame-supported and fluid-supported domains. Different rocks have their particular critical porosities. Sands within the wormholes are suspended within mixed fluids, including heavy oil, water and foamy oil.