Mitigation of Deepwater Shallow Hazards with Simplified Cement System: South China Sea Case Study

Cementing is one of the most critical steps in the well construction phase. Qualified cement integrity is considered a barrier to inhibit fluid migration to surface. For conventional wells, cement in addition to barriers such as BOPs (Blow Out Preventers), fluid hydrostatic pressure, and mechanical methods are used to ensure fluid migration is inhibited. For top-hole sections of deepwater exploration wells however, cementing is your only barrier to mitigate uncontrolled fluid flow to the seabed. During the top-hole operations (typically 36-in. and 26-in. hole sections), the bore hole is drilled without a riser and returns are taken to seabed. This creates difficultly in controlling shallow hazards such as gas hydrates and shallow water flow because the well cannot be killed using conventional methods. To assure mitigation of shallow hazards, an emphasis is made on achieving short transition times for cement designs. For South China Sea operations, it is common practice to incorporate cenospheres (dry blended lightweight additive) into the cement design to ensure suitable properties are achieved. However, the use of cenospheres requires blending onshore and excess blends to be shipped offshore for contingency purposes. Although required slurry properties can be achieved with cenospheres; logistical issues, left over disposal concerns, costs, and low cenosphere pressure ratings create a complex operation. In addition, cement systems should be designed to incorporate loss of lightweight material during transfer offshore, and must account for higher down-hole densities due to crushing of lightweight cenospheres. All of these processes require great emphasis on Quality Assurance / Quality Control (QA/QC) and economic concessions. Other unique challenges that must be accounted for are low temperatures, commonly around 4°C (50° F) at seabed, low fracture gradients, enlarged hole sizes due to high rate of penetration (ROP) drilling, and environmental regulations due to returns at seabed. The cement system must be designed for all the above aspects, and still maintain excellent integrity to enable the BOP stack to be installed upon a firm foundation. This study will describe a new simplified cement system which exhibits excellent slurry properties necessary for a deepwater environment. Laboratory results and engineering procedures will be reviewed to demonstrate that all necessary qualities for mitigation of shallow hazards are met. Furthermore, logistical and HSE advantages will be highlighted, and two case histories will be discussed.