Cave Geomorphology and its Effects on Oil Recovery Factors in Tarim Karst Reservoirs, West China
Authors: Montaron Bernard A , Xue Fangjian Jack , Tian Wei , R. Han Ray P. 
Published: May 2016

Abstract
Tarim carbonate reservoirs have been produced by Sinopec NW and Petrochina Tarim Oil Company since the late 1990's. Oil production from Tarim carbonates gradually increased during the last 15 years to reach close to 9.5 million tons (67 million bbl) in 2012, with an estimated 83 million ton total - almost 600 million bbl - extracted from the beginning. The oil extracted from these deep karst carbonate formations is contained in cave systems, and the dynamic and recovery behavior of these reservoirs is completely different from conventional matrix reservoirs.
The estimation of recoverable reserves in this type of reservoir is a major challenge. The determination of volumes, fluid saturations, and recovery factors is equally difficult for the three. Regarding volumes we discuss here some interesting consequences of the statistics of cave geometries, but a lot of work remains to be done in order to develop a practical method for volumes and average saturation determination.
The main contribution of this work is the development of a theoretical method to estimate oil recovery factors in karst reservoirs based on the modeling of the geomorphology of caves. According to this model buoyancy forces trap the oil in a multitude of attic pockets in the ceiling of caves. The trapping of the oil is the consequence of the extremely tight matrix, with zero permeability to oil, surrounding the open caves in Tarim Ordovician carbonate fields. The parameters of the mathematical cave model were calibrated using a 3D LIDAR survey of Shihua cave in an Ordovician carbonate outcrop near Beijing. More than five hundred random caves were generated on a computer and calculations of the volume of attic traps were carried out to determine oil recovery factors as a function of various parameters, such as the initial water level in the caves, or the position of the producing well.
This work shows for the first time that oil recovery factors in cave systems of Tarim basin can be surprisingly low despite the fact that oil is not capillary bound to the rock. This new understanding of karst reservoirs led to propose a new oil recovery method: using nitrogen as a displacement fluid. Nitrogen foam EOR has the potential to at least double oil recovery factors in Tarim carbonate reservoirs. The additional amount of oil that could be extracted from TaHe field alone could exceed 115 million tons (800+ million bbl). If applied to all Tarim carbonate fields the incremental oil production could exceed 200 million tons, i.e. well above one billion barrels.

Oil production history in Tarim basin carbonate oilfields
Tarim basin is located in the Xinjiang province in West China. Tarim carbonate reservoirs have been produced by Sinopec NW and Petrochina Tarim Oil Company (TOC) since the late 1990's. Oil production from Tarim carbonates graduallyincreased during the last 15 years to reach close to 9.5 million tons (67 million bbl) in 2012, with an estimated 83 million tontotal (almost 600 million bbl) extracted from the beginning.
Petrochina TOC is developing several carbonate fields including Tazhong in central Tarim, and Lungu field in the North. Sinopec NW is exploiting the TaHe oilfield which represented more than 75% of the total oil extracted from Tarim carbonates since 1997 (Fig.1).
There are signs that TaHe field production is getting close to peak, and it will be difficult to avoid the decline of production using current field development methods. But finding new ways to increase production requires a good understanding of the reservoir production and recovery behaviour, which is the key objective of the work presented here.