RIASSUNTO
Abstract
A new generation of multi-lobe positive displacement mud motors designed in the USSR has been used in Europe since early 1981. A comprehensive range of experience has already demonstrated the importance and potential of this development for a wide variety of drilling applications. This paper describes the design concept of the motor which operates on the Moineau principle and discusses the performance with principle and discusses the performance with reference to a number of recent applications including straight hole drilling, directional drilling and coring. The slow-speed, high-torque characteristics, which permit sustained high bit weights, give near rotary conditions at the bit for the first time with a downhole motor, offering many advantages in drilling.
The references to case histories include a summary of the comprehensive use of the motors for directional drilling in two 7000 ft deep wells in granite for a hot dry rock geothermal energy research project in Cornwall, UK. In addition comments on the use of the motors with polycrystalline diamond and tricone bits for straight hole polycrystalline diamond and tricone bits for straight hole drilling are presented, together with a number of references to coring applications.
The results so far demonstrate that longer runs, faster drilling, and more successful coring are possible and that the power-speed characteristics offer new important operational options including bottom hole configurations with drilling assemblies below the motor. The walk tendency of the motors in directional drilling is opposite to that of turbines and, therefore, complements turbines for long progressive corrections in drilling tangent sections.
The technical improvements to the original design based on early operational experience are outlined and future potential applications discussed.
Introduction
Since March 1981, a new generation of positive displacement downhole motors has been available from a base in Aberdeen, Scotland principally for onshore operations in Western Europe and offshore service in the North Sea. Operations to date have included oilfield and hard rock drilling. In the first year, 12 motors in two sizes, 6 3/4 in OD and 7 5/8 in OD, have been used for straight hole drilling, directional drilling, coring, milling and cutting casing. The experience has been used to refine the design and define manufacturing parameters for a new range of motors which are parameters for a new range of motors which are entering large scale manufacture.
The granite drilling undertaken by Camborne School of Mines, Cornwall, was particularly significant both from the standpoint of the success achieved in drilling hard, abrasive rock and the contribution to the improved design and development of the motors.
A wide variety of bits have been used which have demonstrated that rotary drilling parameters at the bit generated by a low-speed high-torque downhole motor unit can give substantial benefits over alternative tools and methods.
The paper describes the experience over the first 12 to 15 months of operation.
MOTOR DESIGN BASIS
These multi-lobe positive displacement motors were evolved over a development period spanning 10 years. The objective was to generate output characteristics equivalent to those of a rotary table. High torque at rotational speeds less than 220 rpm with power being applied at the bit enables the motors to be used within the scope of standard drilling practices including existing bit and mud systems technologies. The technical data given in this section refers to the Drilex 6 3/4 in OD motor.