Two-Dimensional Permeability of Breccia Facies in the Athabasca Oil Sands

Abstract

Abstract Breccia is a lithological facies present in the McMurray Formation in the Athabasca region of northeast Alberta. It consists of mud or shale clasts embedded in clean sands. The mud content of this facies varies from 0 to 100% and the clasts range in size from millimetres to metres with varying degree of angularity and sorting. Breccia is common in bitumen reservoirs. From an in situ recovery perspective, breccia can impede fluid flow and reduce the overall oil saturation due to the presence of the mud clasts. To properly characterize the reservoir flow properties, it is necessary to develop a systematic procedure to capture the impact of these clasts to a scale that is suitable for numerical flow simulation. This is commonly referred to as upscaling in reservoir characterization. The procedure described here involves digitizing core photographs of breccia intervals and assigning a sand-mud facies indicator based on the presence or absence of mud clasts at the pixel level of the core photograph. The resulting indicator map is transferred to a numerical flow simulator. The 2D effective permeability of the breccia zones is determined numerically from single-phase flow simulations by assigning a constant permeability to the sand and zero permeability to the mud clasts. The results indicate that breccia permeability closely follows an averaging power law in terms of the mud content. A 2D mathematical theory is developed to calculate the permeability of breccia facies by treating the mud clasts as elliptic obstacles to the flow through the embedding sand of known permeability. The results from the theory are shown to be in satisfactory agreement with those from the flow simulation. The paper concludes with guidelines for systematic permeability assignment in the breccia zones of oil sands, as well as other similar formations. Introduction Fluvial/estuarine channel sandstones and associated lithofacies host the oil sands deposits of the lower Cretaceous McMurray Formation in the Athabasca region of northeast Alberta. Channel deposits occur where there has been incisement into pre-existing sediments, creating accommodation space for subsequent deposition. Mud clast breccias are commonly associated with these channel deposits. They are mechanically formed through the cutting and bank collapse of the pre-existing sediments as the channels migrate across the depositional area. These breccias are a product of an active energy system. As the channel sand bars migrate, 'ripup' mud clasts that were eroded during the channel action become embedded. The mud clast breccias can vary considerably in length, width and height. They can also vary considerably in the amount of mud clasts in the host sandstone matrix. Mud clast breccias display favourable characteristics to be included as part of a reservoir unit. They are permeable as is evident from the fact that oil migrated into the sand portion of the facies through and around the mudstone clasts. Analysis of core samples reveals the bitumen saturation in the sandstone fraction of the breccias is equal to the bitumen saturation in mud-free sandstones.