An Analysis of the Design Loads Placed on a Well by a Diverter System

Abstract

SPE Members *IADC Member Abstract Diverter systems, used as an alternate means of well control while drilling surface hole, have a demonstrated history of failure. Of particular interest in this paper are failures related to excessive backpressure, such as mechanical failure of surface equipment or the loss of the rig due to foundation collapse. Critical flow effects, neglected by current design practices, are shown both experimentally and theoretically to have a significant effect on backpressure and by including fluid acceleration pressure losses in backpressure calculations. "Systems analysis" of diverter operations is developed and applied to a field example, proving its effectiveness as a design method. In addition to incorporating critical flow effects, this analysis also considers wellbore and reservoir performance. Calculations methods developed to perform the systems analysis are discussed. Design procedures culminate in a method by which diverter vent line diameter, conductor depth, and drilling depth are related to identify a safe interval which to operate the diverter system. Introduction Diverter systems (figure 1) have been used since the early 1970's as a means of well control while drilling the shallow part of a well, when a kick cannot be controlled by shutting-in the well because of unfavorable fracture conditions. The choice therefore is to allow the well to flow using the diverter system rather than attempting a shut-in. Diverter incidences are infrequent, but Mineral Management Service records indicate a failure rate at greater than 50 percent (personal comm., MMS). These failures were mechanical, erosional, or of a broaching nature. Although many of these diverter system failures can be attributed to poor maintenance, a substantial percentage of the failures are attributed to improper design. This study is an at tempt to provide diverter design criteria by linking the reservoir performance, the wellbore performance, and the diverter line performance into a single hydraulic system, utilizing the "Systems Analysis" approach (Crouch and Pack, (1980), Brown and Beggs (1977), Clark and Perkins, (1980) for gas and gas-water flow. This approach requires the calculation of pressure drops as a function of flow rate in the reservoir, in the wellbore, in the diverter line, and at the diverter exit when the flow is critical (sonic velocity). Consequently, the flow rate realized is to the simultaneous solution of these pressure drops with both the reservoir pressure and exit pressure as boundary conditions. These calculations finally allow the engineer to investigate the effects of such parameters as diverter line diameter and length, wellbore diameter and length, conductor pipe setting depth, gas or gas-water flow, and reservoir characteristics. Through this type of simulation, one can arrive at an optimum diverter diameter size as well as study the effects of conductor setting depth. Conductor depth has been found in this work to be a very important parameter, but yet has been practically neglected by drilling engineers. This study was conducted in two parts. First, experimental work was conducted in several pipe sizes to measure exit pressure as critical velocities for gas and gas-water flow as well as model the accelaration effects at critical flow conditions. In this paper, the experimental work presented by Beck, et. al., (1986) for 1 in. and 2 in. model diverter lines is supplemented by data recorded on a 5 inc. diameter, full-scale model diverter line. P. 687^