Abstract

Lost circulation due to induced fracturing is a costly problem in the petroleum drilling industry. After a discussion of the processes of fracture initiation and propagation, this paper reports a case from the Norne field offshore Norway. A production well suffered lost circulation during drilling at a mud density significantly below what had been perceived as the local fracture gradient.

Striking similarities were found between the lost circulation incident and the results of an extended leak-off test that had been performed at the same depth in an offset appraisal well just months earlier. Both datasets indicated that the formation at the relevant depth had a lower-than-prognosed minimum in situ stress, but a high fracture initiation threshold. This additional barrier to lost circulation was irretrievably lost once a fracture had initiated and penetrated a certain distance away from the borehole.

Based on this understanding of the nature of the problem, and an updated set of operational constraints, the drilling operation progressed to the prognosed total depth of a well section that could otherwise have been lost. Future wells will be based on an updated set of pore pressure, collapse pressure, minimum stress and fracture pressure gradients that were established after the incident.

The case illustrates that the conventional methods of determining the pore pressure and fracture gradient may carry considerable uncertainty.

Introduction

Lost circulation due to induced hydraulic fracturing is a costly problem in the petroleum drilling industry. Drilling fluid densities and casing setting depths are often dictated by the fracturing pressure of the penetrated formations. Particularly in offshore production wells, where cost-cutting and extended-reach ambitions are important drivers, well plans often leave a narrow operating window between borehole collapse and lost circulation. Doing such optimization in a safe way requires a thorough knowledge of the local fracture gradient.

The fracture gradient, or limiting pressure gradient for avoiding lost circulation, is traditionally determined by a leak-off test (LOT). In this test the borehole immediately below the casing shoe is pressurized until fluid leak-off becomes noticeable in the pressure-volume curve, indicating that a fracture has been created [1]. The leak-off pressure (LOP) is interpreted as the first deflection from a linear pressure-volume curve. If the local fracture gradient is sufficiently well known at a certain casing shoe depth, a simpler formation integrity test (FIT) is often performed instead. In the latter test the borehole is pressurized to a predetermined value without creating a fracture, thus verifying cement integrity.

A compilation of LOT's (and even FIT's) is often used to generate local and regional depth trends, and thus the predicted fracture gradient for future wells. However some pitfalls exist in using leak-off tests to generate a fracture gradient curve. Individual tests may be difficult to interpret, when no clear or unique deflection point exists. Test data is often recorded manually at a coarse sampling rate, disallowing raw data scruitiny. Some bias towards higher interpretations may even be introduced by the drilling team's eagerness to drill ahead. LOP's from a group of neighbouring wells are often scattered, giving considerable room for subjective interpretation of the local trend.

Theory of Fracturing and Lost Circulation

In order for lost circulation by induced fracturing to occur, two criteria will have to be fulfilled:

  1. The fracture will have to initiate at the borehole wall and grow in the near-well region.

  2. The fracture will have to propagate out of the near-well region and grow to a large surface area.

In this distiction the near-well region is taken to be the volume of rock whose stress state is affected by the presence of the borehole, typically 1–2 hole diameters into the formation.

Theory of Fracturing and Lost Circulation

In order for lost circulation by induced fracturing to occur, two criteria will have to be fulfilled:

  1. The fracture will have to initiate at the borehole wall and grow in the near-well region.

  2. The fracture will have to propagate out of the near-well region and grow to a large surface area.

In this distiction the near-well region is taken to be the volume of rock whose stress state is affected by the presence of the borehole, typically 1–2 hole diameters into the formation.

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