--> Novel Insights to the Unconventional Reservoir: Unlocking the Mystery of GOR Variations

AAPG ACE 2018

Datapages, Inc.Print this page

Novel Insights to the Unconventional Reservoir: Unlocking the Mystery of GOR Variations

Abstract

A key aspect to the unconventional reservoir definition is whether or not the rock permeability and/or fluid viscosity needs to be altered to achieve a commercial flow rate (e.g., Candor, 2012). As viscosity is usually assumed to be static, the alteration is usually applied to the rock matrix by virtue of fracking. Using a pivot logic perspective, the focus is redirected to the identification of variables in the hydrocarbon phase encountered in the pilot hole (e.g., to plan the landing) or lateral that are associated with lower viscosity hydrocarbon fluids (i.e., higher gas-oil ratios). Examples of petroleum system processes that lead to higher GOR include auxiliary hydrocarbon charge that can be migrated or generated in-situ, whereas the inverse scenario is often accomplished with a leaky top seal.

There are several strategies to unlock GOR variations. An initial assessment of relevant trends can be achieved with a data ferret, often within the public domain. When the project advances to drill stage, the best practice utilizes a wellsite mass spectrometer. This is attributed to the ability of this instrument to understand the distribution of molecules that exert control on reservoir energy within penetrated rock in real time. This is accomplished by deconvolution of the collective mass spectra to determine hydrocarbon (e.g., gas, condensate, and oil) and non-hydrocarbon (e.g., helium, hydrogen, acetic acid, carbon dioxide, hydrogen sulfide) components in the drilling mud system. Critical parameters directly determined include total hydrocarbon signal, gas-oil ratio, porosity and volatility, and water saturation and mobility. Interpretive extensions include initial assessment of top seal efficiency, bit wear (i.e., without a trip), determination of optimal landing, completion design, geosteering, and suitability for enhanced oil recovery. When integrated with more conventional methods in the geochemical toolbox, critical components of the petroleum system can be identified and quantified, such as indigenous versus migrated hydrocarbons (i.e., latter is more common than implied by the prevailing paradigm, especially in hero wells / fields), moveable oil can be quantified, and top seal efficiency determined (i.e., critical role to understand under-performing wells). Dramatic operational cost savings to the entire resource development are demonstrated with case studies in the Wolfcamp (Texas), Bakken (North Dakota), and Mancos (New Mexico) petroleum systems.