"Don't Let the Temperature Log Fool You": False Indications of Height Containment from Case Studies in a Tectonically Stressed Environment
Authors: Dozier George C. 
Published: February 2016

Fracture height prediction and evaluation is critical in understanding the effectiveness of a fracturing treatment. Volumetrically, fracturing must adhere to mass balance equations. Therefore, proppant placed in the fracture must be accounted for in the creation of fracture height, width, and length. In many cases, excessive fracture height generation is at the expense of fracture width and length creation. As a result, in fracture treatments where excessive height growth is believed to have occurred, premature screenouts are usually the result of insufficient fracture width. This unfortunate circumstance creates an operational strain and productivity underperformance for wells.

Various methods have been used to evaluate fracture height prior to the fracture treatment. These methods can be as simple as height estimates based on sensitivity studies of fracture height growth for different fluids, pumping rates, etc., using a derived rock stress profile within a fracturing simulator to the most robust methods of direct measurement using passive seismic monitoring of fracture events during an injection test. As the industry comes closer to what is believed to be direct measurement of fracture height, the level of trust increases as well.

Both temperature logs and radioactive tracers have been used on a regular basis by the fracturing industry to evaluate fracture height containment (or the lack thereof). However, the information from these tools may be quite misleading when planning a fracture treatment due to the inherent assumption made when analyzing the log data. The major assumption is of a vertical fracture propagating from the wellbore. Nevertheless, in a tectonically stressed environment where the maximum principal stress is not always an overburden, the assumption of a vertical fracture may be incorrect. Where this is true, the indication of height control seen in these log types may be a mirage. This paper describes, through case studies, the unique problems of evaluating fracture height in tectonically stressed formations. Furthermore, it shows that while both temperature and tracer logs may add value to evaluating fracture effectiveness, their results should be validated by a thorough pressure analysis of the injection data for validity. If this critical step is forgone, many assumptions of height control, as indicated by a temperature log alone, may in fact conceal an excessive height growth environment and lead to premature screenouts.

Introduction

Much of Algeria's future oil and gas recovery will depend on its ability to effectively use fracture technology to improve the production (eliminate skin) of its existing moderate permeability reservoirs (ranging from 5.0 - 200mD) and unlock its large reserve base in the low-permeability (0.001 - 1mD) Cambro-Ordovician reservoirs, specifically the Quartzite of Hamra. However due to the tectonic history of North Africa, hydraulic fracture design, execution, and evaluation is neither simple nor straightforward. On the contrary, this paper will illustrate that depending solely on the results of a simple temperature survey can often lead to misinterpretation of data and, subsequently, inadequate preparation for treatment success. The key to success is in applying multiple techniques that can help validate a particular observation from different vantage points.

The case in point is hydraulic fracture height, and can it be effectively assessed using temperature logs to determine height containment in a tectonically stressed environment. This paper will illustrate several case studies where solely temperature logs have been used, in a tectonically stressed environment, to determine fracture height. The resulting temperature survey will show ambiguous results under common interpretation techniques, often leading to misinterpretation. The paper will then discuss theorectical data that suggests that additional information can be surmised from these ambiguous logs. Lastly, injection diagnostic pressure information will be analyzed on the same wells to determine whether further information can be gathered on height growth. Through combining the different information sources, we hope to shed a much clearer picture on the actual treatment results1.