ABSTRACT

It is usually difficult to balance the contradictory requirements for pumping displacement performance and long-term sealing mechanical performance of high-density cement slurry. To achieve a good quality high-density cement slurry, cementing performance is based on balancing the requirements for pumping displacement performance and long-term sealing mechanical performance. The close packing theory was used to design a high-performance high density cement slurry, which can effectively solve the formula design problem of high-density cement under complex well conditions. In this paper, the weighting materials suitable to close packing design are selected by testing the surface properties and particle size distribution of particles. Based on the close packing model (CPM), the packing density of the multi-component particle system was predicted. Combined with the prediction of water demand, a new method to design high-density cement slurry system was formed. Through the method, the formula of cement slurry based on the cement, BCW-500S and ilmenite powder ternary system was calculated, and the performance of the cement slurry was measured. The testing result shows that the ternary cement system (2.40g/cm3) with close packing and optimized design has good rheological property (n≥0.8, k≤0.6Pa.sn) and high compression strength (>30MPa, 48h), the contradiction between rheological and mechanical properties of cement paste is balanced effectively, and the comprehensive performance meets the requirements of field cementing for Libra Oilfield.

INTRODUCTION

The Libra Oilfield in Brazil is a recently discovered, world-class deep-water sub-salt oilfield. The gas and oil wells in the Libra oilfield generally must be developed through thick salt-gypsum layers with a thickness of approximately 633∼2537m. The geological conditions of the salt-gypsum layer are complicated and the drilling and cementing is extremely difficult. Due to deep burial and rapid creep of the salt layer, it is easy to cause casing collapse. The saltwater pressure is high (2.30∼2.40g/cm3), leakage pressure is relatively low (2.41∼2.45g/cm3), the cementing safety density window is extremely narrow (as low as 0.07g/cm3), and leakage and overflow are prone to occur during cementing. In high pressure salt layer cementing, high density cement slurry is required to have good rheological property to meet the displacement requirement of narrow density window. Furthermore, good mechanical properties are also needed to prevent the risk of casing collapse by the creep of salt layer. However, it is usually difficult to balance the contradictory requirements for the two properties, especially for the development of high density cement slurry (Huang, 2001).

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