Abstract

Lost circulation is a major and costly drilling problem that severely affects the drilling curve and increases costs. To address this problem, a new treatment system has been developed that is pumped through the bit, for accurate placement into the lost-circulation zone. Such a plugging fluid, designed to gel after being sheared through a bit, cures the losses without the need for extra drill pipe trips, drilling of cement plugs, or time spent waiting on cement. The quick rate of cure allows operators to resume drilling with minimum downtime, usually within 30 minutes of a successful treatment.

Introduction

This paper describes the technology and performance of the system and presents field case studies that demonstrate its success in curing problems with total loss of circulation. In most cases, drilling was resumed in less than 1 hour, with total circulation, after successful treatment by this innovative lost-circulation technology. This system is called Shear Sensitive Plugging Fluid (SSPF).

CONVENTIONAL LC TREATMENTS

Lost circulation (LC) is major drilling problem that can be very detrimental to drilling practices and the economic curve. There are many causes of LC and various techniques for solutions. Techniques, in the past have included thixotropic cements, diesel oil bentonite plugs, dual injection plugs and LCM sweeps. Such methods of LC treatment have limited application, some more promising in minor losses and weak formations, others for vugular and fractured formations.

LC can be a predictable problem in a developed field and contingency plans are often evaluated. Many operators can accurately predict the depth at which losses will be expected and the rate at which drilling fluid will be lost. One developed field, Chinchaga, in Northern Alberta, is such a case. If losses occur in this field, it will be at a depth of 900 m to 1050 m, in the top of the Debolt formation and will usually be total. The history of this field has had limited success with traditional LC treatments and require alternative solutions.

Thixotropic cements have limited capability and require precious rig time. The success of cements depends on the thief zone characteristics. Large fractured, vugular formations are unable to support a 1700 kg/m3 plug with a thickening time of 90 minutes. The large, deviated, voids do not slow the slurry sufficiently to demonstrate its thixotropic characteristic. In addition, the time required to trip drill pipe out of and into the hole, waiting on cement and drilling out, is not cost effective for operators.

The placement of LCM sweeps is a traditional solution that has plugged zones with low success rates. LCM plugs will often deteriorate over time due to tripping pipe and increased ECD across the treated zone. LCM's can occasionally plug drill nozzles requiring extra tripping or extreme well control methods if LC and kicks occur simultaneously.

Dual injection fluids that inter-mix downhole at the LC zone depend on many factors to be successful.

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