| Popis: |
Typically, the position of the top cementing plug in a subsea casing string cementing job is approximated based on the calculated casing volume, however, for various reasons, this volume often varies significantly from actual string volume, especially in long, heavy subsea casing intervals. These discrepancies can create significant challenges during primary cementing operations: If the top plug is higher in the string than anticipated the cement displacement ceases prematurely, preventing the top plug from bumping. This technique prevents over-displacement of cement, but leaves excess cement in the shoe track and increases drillout time.Conversely, the top plug may be lower in the string than anticipated, resulting in the plug bumping unexpectedly at a high displacement rate. This can lead to plug bypass, subsequent over-displacement and a mixture of cement and mud in the shoe track. Operators using dual cementing plugs have attempted to avoid displacement errors by recognizing the volume required to bump the bottom plug. This method has proved to be unreliable because of the freefall effect of cement in the casing string and, in most cases, rupture of the bottom plug can neither be observed nor recorded. A new plug indicator system (PIS) has been developed which addresses these problems by providing a positive indication on the rig floor of the top plug’s exact position in the casing string shortly before the plug will bump. This indication is produced by a specialized sub installed in the casing string above the plug’s landing surface. As the top plug passes through the sub, a brief increase in displacement pressure is recorded at surface. This enables the operator to confirm the plug’s exact position in the string, mitigating the risk of cement over-displacement and enabling controlled bumping of the plug, optimum cement placement and minimum drillout time. This paper addresses the problems associated with accurately locating a top cementing plug and describes the principles of operation of the PIS and its deployment in subsea casing-string cementing. This paper will provide details of the experience gained from the use of the PIS in the field, with a detailed case study of a deep, subsea Gulf of Mexico well. |
