Wells in the northeastern United States are generally drilled to a depth of from 3,000 to 6,000 ft. They are usually air drilled through several incompetent formations among which are the Marcellus and Coffee shales. Completions in this area are hampered by very low fracture gradients of 0.4 to 0.6 psi/ft, with most of the formations containing a large number of natural fractures. During cementing, pressures in excess of 1,100-psi hydrostatic can result in breakdown of the formation leading to incomplete fill-up on the cement job.
This paper will discuss the existing completion practices in this area, which include the use practices in this area, which include the use of multistage cementing, and the incorporation of cementing baskets and other downhole tools. The current cement systems in use and the problems encountered in using them will also be discussed.
Several case histories of new cementing techniques, using ultralight weight foam-cementing systems, will be presented along with the job design used on these wells.
Bond logging of the foam-cemented wells creates an array of special problems for the logging companies, due to the ultra-low densities and the high porosities of these special cementing systems. Newly developed techniques for logging these wells will be discussed, along with the bond logs from the case histories.
Many wells in the northeastern United States are air drilled through several incompetent formations. Fracture gradients can run as low as 0.4 psi/ft. Many operators have been very successful in cementing these wells using stage cementing practices. This paper will address the areas where, despite stage cementing, cement fill-up cannot be achieved. Discussion will center around the problems associated with the existing cementing practices, as well as the alternatives to these. Case histories of wells completed using ultralight weight foamed cement are presented along with the cement bond logs from these wells.
A brief discussion of methods to properly evaluate these logs is also included.
Cement job design in these wells has generally been centered around combating the very low fracture gradients, while attempting to obtain the maximum cement fill-up possible. Wells are classically cemented using stage collars in conjunction with from three to seven cementing baskets. Often, the operator is faced with either doing several remedial squeeze jobs in an attempt to cover the entire zone, or only covering the zone of interest while leaving the remainder of the hole uncemented, which can lead to excessive corrosion of the casing.
Cement system designs used on these wells normally use thixotropic cement systems with large quantities of lost-circulation materials. Either the bottom and top stages are cemented using this system, or the bottom stage incorporates this system, while the top stage is cemented using an extended cement system such as 50:50 blend of cement and pozzolan.
This type of approach can cause an array of problems. While all stage collars are designed and manufactured to operate properly, all too often they do not. Costly rig time is also lost while waiting for the bottom stage of cement to obtain sufficient compressive strength to continue the operation. The stage tool must be subsequently drilled out before continuing with the stimulation treatments, again resulting in additional lost time. Another problem is that when the tool is opened, the displacement fluid used during the first stage can wet previously dry formations which can result in excessive formation damage in the producing zones, or the breakdown of weak zones.