Drilling Fluid: Lost Circulation One of the major causes of down time in the drilling industry is Lost Circulation. It's so wide spread that hardly a well is drilled without some form of loss. The cost to the industry runs into telephone number and far more than I will ever see  "Lost circulation," or "lost returns," means the partial or complete loss of the drilling fluid to voids in the formation. Such losses occur under varied conditions and the cause can often be difficult to identify.  Lost circulation is common in drilling horizontal wells in naturally fractured reservoirs. Many of the wells  drilled required weighted mud's or brine. Horizontal wells drilled  in areas of substantial depletion and in  areas with a high gas pressure (>14 ppg) gradient. will often find mud issues often dominate the economics of drilling. In the planning of any well there are consideration that have to be taken into account. The drilling fluid being a top priority. Such consideration as The nature and type of formations to be penetrated: The site, as affecting accessibility to supplies. The primary purpose of the operation Limitations, as well as the capabilities of the drilling equipment and mixing system and disposal of wastes need to be addressed  Another area to look at is experience and attitude of the drilling crew. Careful attention should be given to problem area, There is no such thing as it cant happen, It can and will if you stand back and let it. planning and utilize fully a carefully designed drilling fluids program is being ready. Unfortunately even the best laid plans can go astray and not all drilling problems can be solved by even the most carefully prepared and maintained mud. Subsurface conditions that cause, or lead to, loss of circulation can be classed broadly as:  induced or created fractures (fast tripping, underground blowouts)  cavernous formations (crevices and channels) unconsolidated or highly permeable formations (loose gravels) natural or intrinsic fractures To stop the loss of  drilling fluids, the voids must be plugged so that a filter cake can be formed on the porous section. The plugging material must be of such consistency or contain particles of such sizes as to offer greater resistance of the drilling fluid into the voids than the resistance to movement upward through the annulus.  Avoiding Loss of Circulation The old proverb, " prevention is better than cure," stands fast when it come to the problem of loss  circulation. No other problem in drilling is so dependent upon the practices of the driller. Circulation losses can be avoided in zones known to be troublesome by simply adhering to good drilling practice  Raise or lower the drill string slowly. Ream any tight section;  Don't not spud. Begin rotation and then start the pump at a low rate and gradually increase the rate. Operate the pump at the lowest rate that will assure adequate cooling of the bit and removal of cuttings. If seepage losses occur while drilling, condition the mud before the losses become severe. Prevent bit  and stabilizer balling Do not drill soft formations so fast as to overload the hole. Make frequent measurements of mud properties and treat and  maintain the drilling fluid minimum weight (hydrostatic head and solids content) minimum viscosity (pressure in the annulus while circulating) minimum filtration (filter cake restriction of the annulus)  Keep gel strength down  In other word keep the surge pressure's under control