Casing Running Considerations In Highly Deviated Wellbores

Abstract

Abstract This paper presents the results of a study that was carried out to improve production casing running times in highly deviated wellbores. Inadequate hole cleaning can be a major problem when drilling, casing and cementing high-angle wells. The mechanisms of hole cleaning are discussed, along with case histories from several wells. Recommended operating parameters are presented, as well as benefits that can be realized by following them. Introduction During the early phase of Esso Resources' Expansion Project at Norman Wells, unacceptably long production casing running times were experienced. On Type II, 60-degree, 810-metre wells, average casing running rates were 98.9 metres per hour with 12.7 metres per hour the minimum (Fig. 1). On Type III, 70-degree, 980 metre wells, the average running rate was 61.1 metres per hour with a minimum rate for one well of 10.6 metres per hour. Initial attempts to reduce casing running times on similar wells included picking up drill collars to 'spud' the casing string. Another attempt involved a cathead-pulley-slip elevator arrangement to snub the casing into the wellbore. When these efforts produced no appreciable benefit, a thorough analysis was warranted. Because inadequate hole cleaning was suspected to be the problem, the study concentrated on this aspect. Case histories of 35-highangle wells and current research into cuttings transport in deviated wellbores were examined. Casing Running Mechanism With improper hole cleaning and cuttings carrying capacity, drill cuttings accumulate in a bed on the low side of the wellbore. When casing is run, it pushes the cuttings to the side or ahead as it goes (Fig. 2). This idea is supported in actual practice by the fact that upon reciprocation and working of the casing through tight spots, it would, almost without exception, return to at least its previous position. Although drag is considerable on a 60-degree well, a pure drag effect would not produce the same result, and hence was not considered the problem. Cutting Transport in Directional Wells The actual mechanism for cuttings transport in deviated wellbores has been the subject of several research projects(1, 2, 3). These studies have indicated that the most important factors affecting the carrying capacity of a drilling fluid include the hole angle, annular velocity, fluid viscosity, fluid density, cuttings feed rate or ROP, and annular geometry. Drillpipe eccentricity and rotation also have a minor impact. Under normal drilling and circulating conditions in a deviated well bore, a bed of cuttings forms on the low side of the hole (Fig. 3). The cuttings bed forms quite rapidly because the cuttings have such a short distance to fall, especially at higher angles. As the bed forms, the annular area is reduced and thus the velocity increases. The increased annular velocity improves cuttings transport. A steady state condition is achieved when the rate of cuttings dropped onto the bed equals the rate of cuttings picked up.