Calibrating Fracture Gradients - An Example Demonstrating Possibilities and Limitations
Authors: Wessling Stefan , Pei Jianyong , Bartetzko Anne , Dahl Thomas , Wendt Brett Lee , Marti Srinagesh K. , Stevens James Clifton 
Published: May 2016

The fracture gradient is the upper boundary for the annular mud pressure while drilling and is one important parameter for wellbore integrity. Its calibration using reliable data such as Leak-off-Tests (LOT) and events such as mud losses is essential and can save the operators large amounts of money. Mud losses are often encountered when the annular pressure exceeds the fracture gradient. Such an event can thus be useful to re-calibrate the fracture gradient in real-time in order to circumvent additional losses while continuing drilling. However, analysis of causes for losses and the resulting delivery of proper calibration data have rarely been reported in the past. In most cases, losses were analyzed after drilling, and the information gained from this analysis has been used to plan future wells.

This paper presents an example data set from the deepwater Gulf of Mexico which was used to investigate possibilities and limitations of applying losses for fracture gradient calibration. The data set originates from two wells drilled one after the other. Drilling the first well became challenging after significant amounts of mud were lost into the formation. The well was finally plugged back and a bypass well was drilled successfully applying the experiences gained from the first well.

In this study, downhole drilling dynamics and formation evaluation data were used to test a strategy for using losses for fracture gradient calibration. The strategy includes the detection of losses, the identification of the zones where losses took place (thief zones), the characterization of thief zones, the analysis of annular pressures and finally the modeling and recalibration of the fracture gradient. While detection of losses and identification of thief zones is possible by multiple propagating resistivity data, the thief zones did not show any additional significant specific characteristic which distinguishes the thief zones from the surrounding formation. Consequently, there is no obvious attribute according to which the losses should have been detected in the early drilling phase. Modeling of the fracture gradient was performed using conventional approaches such as the one by Matthews and Kelly. However, these methods do not provide for prediction of a reduced fracture gradient in the thief zones, so that the calibration against the pressure boundary up to which losses do not occur became inappropriate in the formations where no losses were observed.

Concluding, this paper shows the relevance of downhole data in order to properly analyze losses. However, an appropriate model for the fracture gradient which is able to predict losses is still missing. Such a model needs to consider relevant processes described by parameters which can be updated by real-time logs. In particular, the prediction of losses in specific lithologies or depth intervals requires a good knowledge of rock mechanical parameters and adjusted methods to properly calibrate the fracture gradient from observed losses.

Introduction

Mud losses into the formation causing severe wellbore integrity (WBI) problems and immense additional costs are well recognized in drilling practice. The prevention of mud losses helps to reduce non-productive time and is improved by the application of pre-drill WBI models. These models can be created by a comprehensive post-well analysis of previously drilled wells and properly diagnosing the problems encountered while drilling the wells.