Examining the Monetary and Time Costs of Lost Circulation

Abstract

Geothermal drilling environments tend to be under-pressurized and consist of multiple zones of highly fractured and altered material. As a result, lost circulation is more common in geothermal drilling than in other applications. The implications of lost circulation are numerous, but it can lead to a cascade of unwanted drilling and well completion events from which recovery is difficult (although it is desired within the production zone, as it indicates that permeability has been encountered). In many cases, the well bore needs to be abandoned or redrilled, which can quickly put a geothermal project into economic difficulty. In the order of the least time-consuming and expensive to the most, lost circulation mitigation strategies include 1) drilling ahead “blind”, 2) drilling with lower density muds to reduce the static head in the borehole to below the formation pore pressure, 3) adding lost circulation materials (LCM’s) to the drilling mud to plug the formation and regain circulation, and 4) sealing the lost circulation zone with materials (usually cement) that can be drilled out later. In this project, we examine drilling and cost data from four geothermal fields to better understand the relative costs in time and money associated with lost circulation events. These fields include McGinness Hills and Don A. Campbell in central Nevada, Steamboat Hills in western Nevada, and Puna, on the Big Island of Hawaii. To varying degrees, these fields have all experienced problems with lost circulation that in some cases have resulted in tens of thousands of barrels of mud loss, stuck pipe, twist offs, expensive fishing operations, or redrills.