Oilwell/Gaswell Cement-Sheath Evaluation

Abstract

Technology Today Series articles provide useful summary information on both classic and emerging concepts in petroleum engineering. Purpose: To provide the general reader with a basic understanding of a significant concept, technique, or development within a specific area of technology. All of us can readily recognize construction cement for what it is because we see it every day and it looks how it is supposed to look. Even a poor grade of construction cement is recognizable as cement to the naked eye. In our industry; cement is placed in the casing/openhole annulus for two primary purposes: to isolate producible formation horizons and to support the casing. However, we do not have the luxury of looking at the cement with the naked eye to determine its quantity and/or quality after placement. We are forced to rely on the results of measurements from a variety of electronic downhole tools to define the quality and quantity of cement placed around the casing during the primary cement placement. The major problem associated with interpretation of the results of these measurements lies within one's definition of "good" or "bad" cement. All the cement-evaluation tools available today, as well as the service companies that design and run them, are caught in the vicious cycle of trying to define good or bad cement in the oilwell/gas well annular space. This dilemma arises because we, the operators, are asking them to tell us the compressive strength of the cement and whether the cement occupies 100% of the annular space. We cannot accept that the cement does not exhibit as high a compressive strength in the downhole measurements as it does under laboratory conditions. If the cement does not measure up, it must be bad cement and we must "squeeze" to repair it. In truth, when we evaluate a cement in an oilwell/gas well annulus, all were ally need to know is whether cement exists in the annulus (regardless of its strength) and whether the cement occupies 100% of the annulus. We must understand that all we can do is place cement into a section of the annulus that contains neither cement nor particulate matter when we squeeze an annulus to repair a primary cement. If 10-psi cement exists in 100% of the annulus, noportion of it can be removed for replacement by a 1,000-psi cement. If the annulus is packed with settled barite (from the drilling mud) or formation particles, no portion of the particulate matter can be removed and replaced by cement. Only liquids can be removed from the annulus for replacement by squeeze cementing. Acceptance of this basic premise can both simplify evaluation of a cement sheath in a casing/openhole annulus significantly and complicate measurement methods significantly. This leaves us with trying to identify solids or liquids behind the casing, not the difference between 250- and5,000-psi cement.