The use of horizontal drilling to re-enter existing wells has been put into practice. The benefits of horizontal drilling are numerous. 1) improved depletion of reservoir (increased drainage area), 2) reduced water and gas coning, 3) improved production rates. This paper describes an economical laboratory drilling apparatus used in the evaluation of rate of penetration (ROP) for core and cement drilling. Various cement formulations and materials were tested to reduce ROP in order to obtain a successful short radius side-track. The scope of the work for this project includes:
drill press selection, design and modification,
drill bit design,
drilling fluid selection,
study of ROP on cement densification by dispersant,
ROP of cores from zone to be side-tracked,
effect of material composition and concentration on ROP and
effect of material size on ROP.
During this project, relationships of cement sonic strength measurements verses ROP were made for various cement formulations. Procedures were developed for the drilling of cements or cores with the ROP simulator. Drilling fluid selection was determined by solubility data from the core samples from the zone in which the side-track is performed. Drill bit efficiency data was applied to define the corrected ROP of used drill bits. This data was used to evaluate ROP measurements after the drill bits were partially used. The development of the ROP simulator has given a new understanding of what physical parameters control ROP in well cements.
In Saudi Arabia, old wells are being re-entered and completed horizontally to maximize oil recovery. Short radius wells are being planned for these applications. The zone in which the initial sidetracking is taking place is the cap rock (Anhydrite) directly above the Arab-D. (Fig. 1)
Plug cementing often does not get the specific attention required to perform a more successful job. Generally, lab testing on field samples for plugging operations is not done. Heathman et al developed a "Recommended Plug Procedure" to successfully achieve sidetracks in one problem area where 5 to 12 plugs where set for a kickoff. Other cement plugging successes are also documented.
Most well cement laboratories are equipped with standard API testing equipment. These lab equipment and data from these tests are used to help determine physical properties of liquid cements (slurries) and set cements. API testing equipment is good for standard type of testing such as thickening time, fluid loss, free fluid, etc. However, more realistic data can be obtained when the job procedures, sequences, and operations are more closely simulated. Drilling rate of penetration is one parameter that is used by drilling rig personnel to determine hardness of cement. In the lab, the compressive strength (or sonic strength) data are used to estimate the resistance to drilling (hardness). It is usually assumed that a higher compressive strength cement will have a lower ROP. A drilling device was constructed to help understand ROP of set cement. This device was constructed from a manually operated core plugging type drill press. (Fig. 2)
The major goal for this study was to develop a cement that would have a ROP less than that of the zone to be sidetracked. The theory we worked on was "the more resistant the cement is to drilling (ROP) the better chance of successfully steering the drill string".