Models For Headspace Isotope and Composition Analysis: Gas in Place, Permeability, and Porosity Prediction and Fracking Process

Abstract

The models for headspace isotope analysis The cuttings and cores’ headspace Carbon isotope change is still a puzzle to many geologists. The interpretation models are controversial and conclusions are confusing. Some deem the headspace isotope not much value ( (Patent No. US 2008/0147326 A1, 2008); some correlate the change with permeability, i.e. heavy fractionation correlates to higher permeability (Geomark, 2010~2018); some identified that heavy fractionation correlates to the nano porosity developed where TOC and maturity are high (Chatllier). New Model for headspace isotope analysis With experiment results and fluid dynamics simulation, we developed a quantitative model that could let us interpret the Headspace Isotope and Composition (HIC) together, and to backtrack the original gas/oil in place, pressure, nano porosity and permeability. The model successfully identified GIP from an exploration shale gas well, both in the known sweet spots where TOC is the highest at 5% or higher, as well as stealth payzone where TOC is only 2% or lower. The GIP numbers derived from the new HIC model are also significantly larger compared to other models using other GIP models derived from coal bed methane where adsorption plays a vital role, or conventional reservoirs where PVT analysis using wellhead/bottom‐hole pressure and measured porosity are sufficient. This much higher new GIP numbers derived from the new HIC model help to explain why some prolific shale gas plays have funny recovery numbers from well over 50% to 100% or higher using other models. Role of water in storage, fracking and completion With on‐site isotope analysis of cuttings/core headspace, we further identified that fresh fracking water is vital for initiating imbibition process (T. Engelder, 2014), which is a missing vital part of fracking but often overlooked. Through the headspace isotope measurement and model, we noticed that fresh water effectively opened permanent seals in shale gas rocks, releasing gas retained at high pressure as a result of capillary pressure of saturated brine. The dry shale cutting/core headspace could maintain very heavy fractionated 13C1 isotope (13C1 >‐10‰) over very long period of time, i.e. weeks; and with fresh water added, the headspace 13C1 isotope value quickly changed to ‐ 20‰ or lighter along with significantly higher gas released. The results further verified that capillary seal as the overpressure model and water’s vital role in fracking. The practical applications of this water imbibition HIC analysis is to predict the water saturation of the tight rocks, correct GIP, and guide the fracking and shut‐in process during completion.

AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019