What is Coring?
Coring is the acquisition of a continuous column of reservoir formation using a highly specialised bottom hole assembly. Coring provides the only direct method of formation evaluation. The goals of coring differ from well to well but in general the aim of coring is to obtain a representative sample of the formation in as near reservoir condition as possible, taking into account the objectives of the various interested parties. High ROP is not one of the primary considerations for coring, unlike drilling the object is not to go as fast as possible, but to recover as good quality core as possible. The key performance indicators for coring are:
Core Quality – Maximum recovery of core, minimising fluid invasion while preserving the mechanical integrity of the formation for analysis and accurate formation data.
Core Recovery – Maximising barrel efficiency per run and recovering as much core as possible from each core cut.
Efficiency = Core Cut/Barrel length
Coring Time – Fewer Trips with Longer Cores reduces NPT risk and lowers cost.
Porosity, Permeability, Water & Hydrocarbon Saturation, and Grain Density are all important aspects of reservoir description. Although we can estimate these properties from well logs, the only way we can measure them, is through testing on cores. Coring is a fundamental component of Formation Evaluation (FE) providing the actual formation material needed for:
hands-on laboratory testing and evaluation
geological descriptions - lithological and petrographic evaluation
routine core analysis for storage capacity and ability to transmit fluids
advanced rock properties testing for relative permeability, capillary pressure, wettability, acoustic properties, rock strength, etc.
Analysis of core material (core analysis) generates data that is used by Geoscience groups and operators to:
Integrate, interpret, and calibrate all types of well logs – wireline or LWD
provide accurate Rw and saturation values
What can seem to be very small errors in reservoir data can have massive cost implications. Coring provides the most accurate reservoir data minimising these errors, and therefore more accurately reflects reservoir reserves and reduces cost uncertainties.
“Cores provide essential data for the exploration, evaluation and production of oil and gas reservoirs. These rock samples allow geoscientists to examine first hand the depositional sequences penetrated by a drill bit. They offer evidence of the presence, distribution and deliverability of hydrocarbons and can reveal variations in reservoir traits that might not be detected through downhole logging alone. Through measurement and analysis of porosity, permeability and fluid saturation from core samples, operators are better able to characterize pore systems in the rock and accurately model reservoir behavior to optimize production.
Core Analysis is vital for determining rock matrix properties and is an important resource for formation characterization. The process known as routine core analysis helps geoscientists evaluate porosity, permeability, fluid saturation, grain density, lithology and texture. Routine core analysis laboratories (RCALs) frequently provide a variety of additional services such as core gamma logging for correlating core depth with well bore logging depth, core computed tomography(CT) scans for characterizing rock heterogeneity and core photographs for documenting and describing the core.
When operators need to understand complex reservoir behaviors, they turn to special core analysis for detailed measurements of specific properties. Special Core Analysis Laboratories (SCAL) are typically equipped to measure capillary pressure, relative permeability, electrical properties, formation damage, nuclear magnetic resonance (NMR relaxation time, recovery factor, wettability and other parameters for calibrating logs. SCAL services are also used to characterize reservoirs for enhanced oil recovery (EOR) and for studying multiphase flow and rock-fluid interactions.”
Mark A. Anderson et al, Core Truth in Formation Evaluation, Oilfield Review, Summer 2013, 25 no.2
Information from Core can be used to:
Design more effective completion strategies.
Extend the well life.
Remember: Coring is fundamental to formation evaluation.
Physical measurements are carried out on actual formation rock.