1. Introduction

- [German] Okay, so good morning and afternoon everybody. As Sonya said, my name is German Merletti. I'm going to be presenting this paper, which is actually a collaboration between BP geologist and petrophysicist, including Pavel Gramin, Sarita Salunke, Jeff Hamman, David Spain, Vahid Shabro, and Peter Armitage. And also, from UT Austin, Carlos Torres-Verdin and Greg Salter, and John Dacy from Core Lab. So I would like to thank to them for their contribution to this piece of work. So this is the proposed outline. I'm going to first be talking about what is the main issue when trying to predict water saturation distribution with depth in tight-gas reservoirs. Then, the motivation, so why we have good chances of having good agreement between resistivity-derived saturations and the one derived from capillary pressure in the Almond Formation. And I'm going to be talking about how we combine special core analysis for deriving a primary drainage and primary imbibition models. Then, I'm going to show you a couple of examples of how we do the matching between both sources of saturation. And then, I'm going to show you some conclusions and thoughts. So, as an introduction, as many of you may know, tight-gas reservoirs are commonly burial rocks, which is play a very specific burial history, the burial history and diagenesis. And this diagenesis includes compaction, cementation, and grain dissolution. These rocks also display different fluid pressure and hence the consequent saturation history. So the net result of all these things going on in reservoirs is that rocks went through several cycles of drainage, imbibition, re-drainage, and re-imbibition that make the prediction of water saturation distribution with depths sometimes practically impossible. So in the primary drainage process, which is picked by this blue curve, if you can see my cursor, describes how the hydrocarbon fills the pore space and displaces the original water deposition. This process is well-known and is controlled by differences in fluid densities, interfacial tension, reservoir capillarity, and the height above the free water level. When, once the reservoir is charged, you may have either tilting of the trap or the leakage, or you may have some leakage among faults that could create a reduction cluster leading to the gas migration out of the trap. And this is picked by these red curves in this graph. So this creates a situation when, as I said, a situation where imbibition of water into the gas saturated reservoir occurs. So and during this imbibition event, water migrates along the water-wetting films to reduce the overall gas saturation of the pore system. Then, in any pore throat, there will be a critical pressure difference below which the gas phase continuity across the pore throat cannot be maintained. And a snap shut occurs and severs the gas phase connection across the throat. So once the gas phase access to the pore is removed, then the gas remaining in the pore throat is trapped, and this is show by these two yellow circles. Depending on whether you fully charge a reservoir or whether you are just, you just have a reservoir which is partially charged, you may not with different trapped gas saturation, picked by these two points here. So in order to reconnect this gas or re-drain, the controlling pore throat system needs to be re-saturated. This can take a number of ways. Further gas migration and accumulation may occur in geological time, increasing the buoyancy pressure within the system, or you may have, also, a structural tilting can also cause that secondary drainage of gas into the pore system again. And this is not shown here, but probably a re-drainage would follow this path. So the fluid distribution model of saturation versus height above the free water level are usually well-constrained for simple drainage conditions. But when imbibition and more complex re-drainage and re-imbibition occur, the concept of free water level asin which saturation can be break, this is, it's 50 mass lost. As a result, using core-derived fluid pressure measurements and log-computed saturation to break free water level is, most of the time, impossible. So this is the main issues that we face when trying to reproduce water saturation distribution from capillary pressure.