During the drilling of wells, fractures which are created or widened bydrilling fluid pressure are suspected of being a frequent cause of lostcirculation. A study of the variables which are believed by the authors to berelated to fracturing led to the premise that the presence of an effective lostcirculation material in a drilling mud stream would serve to plug smallfractures at the moment they are encountered or created and thereby eliminate apossible cause of lost circulation by preventing fluid pressure from wideningand extending the fractures. A large number of lost circulation materials were classified and tested insimulated fractures and in shallow wells. Granular type materials with agradation of particle sizes were found to be the most effective for plugging.Within the limits of pump ability, the concentration in mud of each lostcirculation material tested was found to be a controlling factor in determiningthe maximum size fracture which could be sealed. A new type centrifugal sorterwas given preliminary tests and found to be capable of concentrating cuttingsfor rejection and reclaiming lost circulation materials. Introduction The control and prevention of lost circulation of drilling fluids is aproblem frequently encountered during the drilling of oil and gas wells. Lost circulation or lost returns may be defined as the loss to formationvoids of the drilling fluids or cement slurries used in the process of rotarydrilling and well completion. The loss may vary from a gradual lowering of mudlevel in the pits to a complete loss of returns. Not to be confused with lostcirculation is the continuous reduction in mud volume caused by the loss of thefluid phase of muds as filtrate and the solid phase as filter cake during thecontinuous filtration process which occurs on permeable formation surfaces. T.P. 3096

An investigation Of the frzctof's af-fecIing the inter-zona[ flow 0] jluids in the taxing-wcl[borcc7mzzthls of an oil or gas well is presented. Laboratory tests revealed that failure 0/ the casing-cement bond allows conununicarifzn of f?uid at pre.~s[[rciflerclltials near 1,000 psi. Pmforaliirg with a /tol-1UW carrier gun does not~Ilat(er or crack the cement sufficiently to allow comtnunicafion through these :racks and does not seriously affect the casing-cement bond failure pressure. The pressure to cause conununica. lion at "the casing-relnent bond CUII be increased to 3,oOO psi by /imizing pipe deformation during cetlrenti% and subsequent operations. Con?ntuUication can be prevented, however, up (0 pressures saficienl to ca143ethe cement to jail or the casing to collapse, by attaching [O (he ca~ing a seal rir~g of defortnable rubber molded between two sleel flanges, When installed, one ffange is attached rig-. idly to the casing to serve as back-up for [kc gasket wkile tbe other flange is movable.To date, these gnsket seal rings have been field-tested in27 wells. Although comtpunicution iras been noted in two wells, inter-zonal ffow has been prevented in other wells in the same area by installing nzore seal rings between the communicating zones.

AHSTRACT Laboratory and field testing of various squeeze cementing techniques and materials revealed that many improvements could be made in squeeze cementing operations. The use of a slowpumping squeeze cementing procedure permitted the control of the quantity (If . cement displaced into a formation and aided in obtaining a high final squeeze pressure. Field testing indicated the desirability _ ,of .obtaining high _.final squeeze pressures, the need for improved formation breakdown fluids, and the necessity of controlling the pressure differential during testing after a squeeze job.

This paper concerns the rupture or breakdown of rock formations as related to drilling, completing, and stimulating production of wells, and comprises data compiled from a study of literature and records of treatment of oil and gas wells, and from tests conducted in bores drilled into rock cores and outcrops of rock. Results of the investigation indicate that the internal pressure to rupture cylinders of rock and to breakdown rock formations surrounding a bore in the earth is dependent upon the extent of intrusion of fluids, the position of bedding planes, the ratio of internal to external diameter. the tensile strength of rock, and magnitude of confining pressure, and is independent of the size of bore, degree of fluid saturation, and temperature of rock within practical limits.It is concluded that the mathematical relationship of pressure in bores and stresses in the surrounding rock must not be limited by the simplifying assumptions of homogeneity, isotropy, and impermeability; that the incidence of lost circulation of drillin~ fluids to induced fractures may be reduced by preventing intrusion of fluids into the small intrinsic fractures along weak bedding planes; and that the magnitude of the breakdown pressure of wells to be treated may be lowered by removal of mud cake. PROCEDUREThe investigation was initiated with a study of the theories pertaining to the rupturing of thick-walled cylinders of homogeneous, isotropic, impermeable material and a study

A successful air or gas-operated percussion tool has been developed and used in drilling nine wells -one each in New Mexico, Utah and Montana; five in Wyoming; and one in Alberta, Canada. Penetration rates with the percussion tool in these wells have averaged 2.5 times faster than those obtained with rotary gas drilling and approximately 5 times greater than rotary mud drilling. Laboratory and field tests indicate that bit footages with rotary-percussion drilling have averaged 1.3 to 4 times those observed with conventional air rotary. Evaluation of laboratory drilling tests and field results shows that, where air drilling is economical, the percussion tool should be used to further decrease well costs. The additional savings should increase the number of wells in which it is profitable to drill with air instead of with liquid-base drilling fluids.