Successful Planning and Integration of a HFHS Lost Circulation Treatment for Challenging North Sea Brownfield Development
Authors: Sanders Mark W , Houston Kirsty Ann 
Published: May 2016

Abstract
Globally, lost circulation is a major contributor to non-productive time. The types of products available to address this problem are many and varied, ranging from simple fibres and particulates to more complex cements, chemical gels, engineered particulates, and high-solids de-fluidizing treatments. Some are designed to be used in the drilling fluid continuously, whilst others are tailored to be placed independently as discrete pills, subject to the loss situation and, in many cases, the economics.
In scenarios where relatively high levels of losses occur, operational decision trees invariably dictate the use of simpler options where possible. However, these are not always suitable. A positive outcome to the difficulties of one challenging Brownfield location is described. The application of a bespoke multi-fibre high-shear strength lost circulation treatment, when combined with sound contingency planning, has made a significant difference to the continuation of the field.
Situated in the Central North Sea, the site depicted is an established infill development centred around a number of existing wells drilled on the flanks of two complex salt diapirs. Given the physical nature of the field, radial faults and fractures are a major concern. To compound the challenge, depletion effects over time have resulted in changes in the stress regime and a weakening of pre-existing natural fractures. Drilling windows consequently are tight (and not always easily predicted) and large volume drilling fluid losses are commonly experienced.
To mitigate the problem, a revised loss management strategy was developed to include the integration of a high-fluid-loss, high-strength (HFHS) lost circulation product capable of dealing with dynamic loss situations involving fractures of at least 3,000 microns. This paper will describe the development, application, and differentiating benefits for the industry of this treatment and detail its ultimate acceptance as a key element of the operator's lost circulation decision tree.

Introduction
The field detailed in this paper is an established Brownfield site located in the Central Graben, UK Central North Sea. This infill development consists of a mix of both producing and water injection wells constructed on the flanks of two salt diapirs, predominant features of the site referred to as 'north' and 'south' reservoir zones respectively. The reservoir is both oil- and gas-producing Paleocene Forties sandstone combined with fractured chalk. For reservoir management reasons, the gas is currently re-injected and, until the introduction of pressure support, oil production was depletion driven. Based upon data from a nearby producing well differential depletion pressures have fluctuated from historical peaks of around 1,850 psi to less than 30 percent of that value today, with reservoir temperatures of up to 133°C (271°F) being recorded (Murray et al., 2014). Three specific wells will be discussed in this paper, identified here as A1, B1, and C1 (Figure 1). All three are essentially horizontal wells in design.
Given the physical nature of the field, radial faults and fractures are of major concern, particularly when drilling horizontally. To compound the problem, the depletion effects experienced over time have resulted in changes to the stress regime and a weakening of pre-existing natural fractures. Drilling windows, the operational drilling pressure enveloped between pore and formation fracture pressures, as a consequence are tight and not always easily predicted. Much higher mud losses have been commonly experienced in recent years and for this reason a better managed equivalent circulating density (ECD) and lost circulation (LC) strategy was required for continued infill drilling programs. A relatively new high-fluid-loss, high-shear strength (HFHS) LC treatment was a major component of this revised management strategy, the inclusion of which ultimately proved highly successful for the operator. This paper will highlight major challenges associated with the infill development, describe key aspects of the renewed ECD and LC management strategies and will discuss HFHS, its function and the part it played in the success of more recent wells.