In the past, properties of cement slurries have been difficult to control when a number of different parameters are specified, This has been most apparent in higher temperature welts, where retarders are used. The cause of this problem can be mostly related to the fact that each different chemical used in a cement blend affects more than one property, causing the ultimate result of increasing one chemical loading to affect two or possibly three areas of measurement. Common API procedures measure parameters such as thickening time, fluid loss, and rheology.
Fracmaster has now developed a group of chemicals that can eliminate the f1uctualion of these properties, and give superb control of cement slurries even in high temperature squeeze applications. One slurry property can now be changed independent to the other properties of the slurry.
A case history is presented showing a coiled tubing cement squeeze at 120 °C where 3 different fluid loss values were specified in otherwise identical slurries. The desired properties were achieved by adjusting the addition of only one chemical which changed the fluid loss without affecting thickening time or rheology.
Cement blends are recommended by cementing companies from requests made by a customer, and by the downhole conditions the slurry will encounter. In order to supply the requested properties it is necessary to after the cement through the addition of chemical additives. Most of these cement additives have the undesired side effect of changing more than one slurry property. This can make it very difficult to achieve the performance targets specified by the customer.
If a customer is convinced that the inherent properties of the chemical additives provide no damaging properties to an existing blend being programmed, then most likely a wide window of specification has been provided. Performance properties that are requested with narrow guidelines, however, can result in an excessively complicated, and expensive blend being recommended by the cementing company especially when high temperatures are involved.
A novel group of cement additives have now been developed to address these problems in cementing systems and provide a number of other desirable aspects to the slurry.
Most cement slurry property measurements are depicted, and performed by cementing companies in accordance to API testing procedures1. The requested areas of measurement are tested, and reported when they match the preset guidelines at downhole conditions. These measurements usually include fluidloss, rheology, thickening time, and free water.
Cement fluid loss is the measurement of the capacity of a slurry to lose water into a porous medium. Fluid loss is measured by squeezing a cement slurry sample against a very fine screen with nitrogen pressure, and measuring the total volume of water forced out of the slurry over 30 minutes, or until all of the water is squeezed out. The API specified designation for a fluid loss value is Q30A, neat cement having a Q30 of over 600ml/30min, and aQ30 of 100ml/30 min or less being considered a low value.
