The Jean Marie reservoir in the northeastern part of British Columbia, Canada, is one of the most active horizontally underbalanced drilled reservoirs in the world. The Jean Marie is a depleted, low permeability, fractured carbonate with low initial water saturation making it susceptible to formation damage using conventional drilling fluids.1 Given these reservoir characteristics, horizontal underbalanced drilling is the current method of choice for operators active in the area.

Although underbalanced technology has made tremendous improvements in maintaining and ensuring underbalanced conditions at all times during operations, overbalanced incidents do occur while drilling the horizontal section. These incidents can be much more damaging than drilling with a well designed overbalanced fluid. In some cases, an overbalanced event could permanently impair production from a well by as much as 75%.

With the aid of real time downhole pressure gauges, these overbalanced incidents can be identified but very often go undetected, leading to a false sense of achieving the goal of minimal damage. The unidentified damage can lead to misinterpretation of production results and disappointment with underbalanced drilling.

To exacerbate the problem of identifying damage, traditional pressure transient analysis of wells in the Jean Marie may yield incorrect valuation of the overbalanced incidents by underestimating the skin on the face of the horizontal well. This could be due to the difficulty of performing meaningful transient analysis on a horizontal well in a dual porosity reservoir and a general lack of knowledge of the impact of drilling related damage on fractured or highly heterogeneous reservoirs.

This paper will show how, by combining transient productivity index data collected while drilling with the use of history matching techniques, overbalanced incidents can be easily identified and the impact on well productivity can be quantified.


Although it is often assumed that an underbalanced drilled well will have minimal formation damage, the application of inappropriate operational techniques or poor bottom hole pressure management can lead to overbalanced incidents and severe damage. Unfortunately for horizontal wells in very heterogeneous or naturally fractured reservoirs, traditional analysis techniques used to determine completion efficiency are not effective.

Pressure transient analysis (PTA) performed on well tests from horizontal wells is typically challenging to interpret, making reservoir characterization difficult.2 The existence of multiple flow regimes, some of which may not be present or obscured by wellbore storage effects, make it difficult to determine accurate reservoir properties. Additionally, the use of a homogeneous reservoir model, which may be good enough to match long-term production performance, makes it difficult to analyze well test data, during which the effects of heterogeneity and multiple flow regimes are most prevalent.

The only flow regime that will directly indicate the skin on the horizontal well is early radial flow. This flow regime is often partially or completely obscured by wellbore storage. The first horizontal line on a derivative plot may not necessarily be the early radial flow regime, but occurs any time there is a transition from wellbore storage to any other flow regime.3 The misidentification of this flat spot as radial flow can lead to gross errors in reservoir property determination. Often, even heavily damaged horizontal wells are typically interpreted with a negative skin, which will be illustrated with real cases in the case studies section of this paper. By definition, however, a damaged well should exhibit a positive skin, not a negative one.

In order to determine the true impact of these overbalanced events, a method that takes into account more information than is typically used in PTA is needed. This paper will present a method to identify and quantify the impact of overbalanced events during the drilling process, present a new method of determining how these overbalanced events will impact upon a well's production and use this information to quantify the negative impact overbalanced events can have on overall well economics.

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