Liquid Cement Premix for Improved Abandonment and Workover Operations
Authors: Rae Phil , Johnston Neil 
Published: December 2007

Many cementing operations, such as well abandonment, workover etc., incur logistical, personnel and operational problems, and costs, disproportionate to the size of the cementing operation. A typical example being offshore well abandonment on a platform which has limited accommodation, no conventional cementing equipment, and where access, space or weight restrictions preclude its installation. This paper outlines the problems of abandonment and remote locations; it describes the concept, laboratory development, pilot testing, introduction and field use of a liquid cement premix. This novel approach helps reduce waste and minimise the environmental impact of the cementing operation and may be particularly suitable where small volumes of accurately mixed slurry are required, for example in coil tubing applications. It is, however, equally applicable for large volumes since the process lends itself to automation. The technique offers radical changes to cementing operations and equipment requirements, and direct savings to the operator through reduction in the costs of personnel, equipment mobilisation, transport, installation, and material wastage.

Introduction

Several thousand oil and gas wells will be abandoned in the next year or so. Many thousands more will be the subjects of workover operations for essential maintenance, stimulation treatments and recompletions. All of the wells destined for abandonment, and a large number of those to be worked-over, will require the use of cement in some part of the operation. Since the first application of cement in the oilfield in 1902, to shut-off bottom water in a California oilwell, to the present day, cementing operations have been conducted with dry cement powder mixed with water (and additives, if necessary) at the wellsite, immediately prior to use.

With the exception of dump bailer operations, small remedial cement jobs are traditionally conducted using identical mixing and pumping equipment as would be used on larger, primary casing jobs. While there have been marked improvements in cementing equipment over the years, the process still involves the use of dry, powdered, Portland cement that is cut from sacks or, more commonly, transferred pneumatically to a mixing unit. Pneumatic transfer involves the use of high capacity air compressors and requires large pressurised vessels (silos) for cement storage. It also places constraints on the geometry, position and general layout of transfer lines and other equipment to optimise free flow of the cement powder. At the mixer itself, the cement is combined with water, and the homogenized slurry is pumped downhole. This may involve the use of complex recirculating mixers often resulting in cementing equipment being very expensive, heavy and having a large "footprint." While space and weight may not be important on most land locations, they are crucial limitations in many offshore, and remote, situations.

A new technique has been developed which dispenses with many of the traditional problems of accurately preparing small slurry volumes, whilst reducing the cost, weight and footprint of the cementing equipment required.

Liquid Cement Premix

The new technique referred to above involves the use of a Liquid Cement Premix (LCP). Liquid Cement Premix can be defined conceptually as a storable cement slurry that can be kept in the liquid state indefinitely and made to set as-and-when required. It consists of a premixed slurry of Portland cement (or other cementitious material) containing set retarding and conditioning agents, with water as the carrier fluid. This storable slurry can be tailored to have a storage life of anywhere from several days to over six months and it can be formulated with standard oilwell cements. API Classes G, H, C and A, from a variety of cement manufacturers worldwide, have all yielded satisfactory LCP's. It is also possible to prepare storable slurries based on blended cements eg. with silica flour, flyash, etc. When required, the LCP is activated, and/or otherwise modified if necessary, to yield a finished slurry with suitable properties for well cementing. This requires the use of an "activator", added immediately prior to pumping the slurry into the well. This activator re-initiates the chemical hydration and allows the slurry to set after a predetermined pumping time.

LCP can be used for all normal well temperatures, eg up to 230 C (450 F), the thickening time is controlled from 1 to 72 hours by the use of activator or conventional retarder.

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