Research A Low Densiy Cement System For The Complex Wells Cementing

with the increasing complexity of drilling geological conditions andprogressive developments in new drilling technologies, requirements for cementing technology have increasingly notable and problems related with cementing job quality have been highlighted. Cementing operations encounter new and more complex challenges due to harsh environments. Cementing problems such as gas or fluid invasion, fluid loss or lost circulation, incomplete annular fill, poor bonding and zonal isolation and over-retardation or prolonged WOC times may encounter in any well at any depth. Specialized cementing additives or systems were usually recommended by service companies to solve these problems, however, some of these products are often cost prohibitive. In some areas, the cement slurry with low density can't meet requirements for cementing across formation with very low fracture gradients, lost circulation and causing casing corrosion. To overcome the challenges of cementing complex well, a new lightweight cement system, of a 120:80 ratio of Class G and fly ash mixed at a density of 130-1.45g/cm3, is presented. In order to improve the use of slurry cement and ash-fly mixed system, repeat experiments were conducted by adding some new cement additives and developed a new stable agent (SSD) which is composed mainly of sepiolite and a new stable agent (TSN). Changing the added SSD and TSN amount can get the low density cement ash-fly mixed slurry system within the range of 1.30-1.45g/cm3. Tests indicated that the low density slurry possess such properties as super strength, adjustable thickening time, better fluid loss control, good mobility and high setting stability, importantly, the cement expand during hardening. It is shown that the low density cement slurry not only meet the need of cementing low-pressure lost-circulation zones, but also enhance bonding between cement and casing/wellbore and improve the quality of well cementing. In addition, the additives of ash-fly and hyperfine silica prevent cement from corrosion, the additives of sepiolite and hyperfine silica make cement slurry display good thixotropic behavior. Severe lost-circulation and squeeze problems can be solved by the thixotropic slurry system. It is very important for preventing casing from corrosion, isolating oil, gas and water from each other and prolonging working life of wells. In addition to, its cost is less, only 65% of the corresponding cement slurry, also, the properties is more/ In a word, the cement system have many excellent properties to solve the problem of cementing complex well with long low-pressure lost-circulation zones.