Integrated Effective Stress Ratio and Stress Path Approach to Fracture Gradient Estimation for Virgin and Depleted Reservoirs
Authors: Okpako Kelvin , David Olayinka 
Published: July 2019

The days of easy hydrocarbon Exploration and Production are fast disappearing in the petroleum business. Today, the industry is saddled with the task to sustain world hydrocarbon resources to meet current and future energy demands. This has resulted in prospecting for hydrocarbon in deep hydrocarbon plays or accumulations at much deeper depths in the Earth's subsurface.

These deep hydrocarbon plays are sometimes below brown fields with significant production from reservoirs that are depleted and below hydrostatic pressure. Exploiting these deep plays requires wells that could traverse these depleted reservoirs, as well as other virgin reservoirs. These plays are often associated with abnormal formation pressures, with reduced drilling margins, defined by elevated formation pressures and fracture gradients. These pore pressures and fracture gradients information drive the well design and delivery (i.e. mud schedule, casing scheme, and rig/wellhead equipment selection). Thus, in well design and delivery, drilling operations should be planned to manage the different pressure regimes and transition zones to be encountered during drilling.

Recently, several Health, Safety, and Environmental (HSE) incidents have been recorded across the globe from well control related incidents that affected lives, assets, and the environment, because formation strength integrity was compromised during oil/gas well operations. One key factor in ensuring that wells are drilled and completed to desired depth without well control incidents is a proper estimation of the formation strength or pressure containment of the open-hole formation during well operations – this is defined by the formation fracture gradient.

The main objective of this paper is to propose a new methodology to estimate fracture gradients based on the application of two different methods for virgin and depleted reservoirs: i.e. the Effective Stress Ratio and the Stress Path methods, respectively. These methods were adapted to the Niger Delta in an integrated modeling workflow as proposed in Reservoir GeoMechanics literature (see ref. #6). Uncertainty management relating to their broad application of this workflow to all onshore/shallow offshore wells/fields has been managed by incorporating a new calibration function to validate the estimated fracture gradients using the actual Leak-Off Test (LOT) data, taken from offset or nearby wells in the formation of interest or its analogue.

In this paper, 120 LOT data from existing wells drilled across 36 onshore fields in Niger Delta were used to determine a correlation between the ratios of the horizontal/vertical effective stresses and depth. This was then used in the Effective Stress Ratio (ESR) workflow for fracture gradient estimation to obtain the in-situ/original fracture gradient of the formation. This defines the static condition and stress state of the formation and sets the scene for the estimation of the current fracture gradient in the Reservoir Stress Path modeling workflow, that considers the dynamic condition/stress state of the formation, providing a more robust and accurate estimate of the fracture gradient, considering post production and time lapse effects. Furthermore, this new methodology is a significant improvement from the use of depth-dependent fracture gradient models to one that integrates field parameters; i.e. LOTs, overburden/in-situ stresses, pore pressure changes, and related rock properties, which describe the geomechanical state of the formation.

This methodology was validated in some oil/gas fields in the Niger Delta and results were compared to measured values obtained from LOT data from these fields. The results compared very well, giving good confidence in the new methodology. Application of this methodology allows prediction of reliable and robust pre-drill fracture gradient values for planned wells. Its significance is in the design and safe delivery of a well, where the well will traverse virgin and depleted formations, in normal and abnormal pressure regimes with large to small drilling windows across pressure transition zones.