Abstract

The cementing of surface casing through permafrost presents unique problems to completion engineers. The major factor is that subsurface temperatures may be below freezing to a depth of 1,500 feet or more. One more requirement for a satisfactory cement job is that the cement must set and develop sufficient compressive strength under these conditions and form an adequate bond with the permafrost formation.

Laboratory tests were conducted with several cement blends under different temperature conditions to evaluate the temperature rise on hydrating, the compressive strength, and the shear bond strength between cement and simulated permafrost. These tests indicated that a gypsum based freezing point depressed cement blend was suitable for cementing through the permafrost zone.

Wellbore temperatures at various depths were monitored at several Arctic wells during cementing operations. These field data provided useful information on the quality of the cement job as reflected by the maximum temperatures attained and the subsequent rate of cooling and the time required to reach peak hydration temperature.

Introduction

The cementing of surface casing through permafrost presents some unusual problems and has resulted in considerable research. The major factor is that subsurface temperatures may be below freezing to a depth of 1,500 feet or more. One requirement for a satisfactory cement job is that the cement must set and develop sufficient compressive strength under these conditions and form an adequate bond with the permafrost formation.

The problems of cementing casing in permafrost were first investigated in 1950 in connection with drilling operations in the U.S. Naval Reserve 4 in Northern Alaska(1). It was reported then that conventional oilwell cements did not set at subfreezing temperatures and that very little hydration takes place below 40 °F. Consequently the temperature of conventional cement must be maintained above 40 °F to ensure a satisfactory set in permafrost. Another solution to the low temperature problem is to use cement blends specially formulated to set in cold environments1,2,3,4. Special blends are available for cementing through permafrost and these are generally referred to as high alumina cements and gypsum based freezing point depressed blend, which is also referred to as a gypsum-cement blend. High alumina cements have a rapid hydration rate and the heat evolved is sufficient to sustain the setting reaction in permafrost. Freezing point depressed gypsum cement blends will set and develop adequate compressive strength rapidly at subfreezing temperatures.

In order to evaluate some of the commercial cement blends that are available for Arctic cementing operations, the thermal behaviour, compressive and shear bond strengths were investigated in the laboratory at different temperature conditions. Wellbore temperature data on several exploratory wells in the Canadian Arctic confirmed some of the laboratory findings.

LABORATORY TESTS

Thermal behaviour, compressive strength and the shear bond strength against simulated permafrost were investigated for conventional oil well cements, high alumina and gypsum-cement blends.

  1. THERMAL BEHAVIOUR

The measurement of temperature in the middle of small cement samples revealed information on the temperature rise during hydration, the time to tile start of hydration and an indication of whether or not a sample had set.

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