Simultaneous Dynamic Killing and Cementing of a Live Well

Abstract

Abstract Drilling with casing is a method of drilling wells using casing as the drillstring. This method has been used in South Texas to reduce well costs. To help reduce well costs even further, underbalanced drilling seemed to have the potential of enabling the elimination of an intermediate casing string. By drilling underbalanced, it would be possible to drill with a mud weight low enough to prevent loss of circulation in the shallow weak zones while dealing with the gas from the pay zones below. Drilling with casing eliminates the necessity of trips, thus simplifying underbalanced operations. However, once total depth (TD) was reached and drilling complete, it would be necessary to balance the well to allow cementing of the production casing string. The challenge was to develop a way to kill the well so that the cement could be placed and allowed to set while at the same time not breaking down and losing circulation into the weak upper zones. This had to be done by having different equivalent circulating densities (ECD) and equivalent mud weights (EMW) up and down the hole throughout the circulating and cementing operation. In addition, this technique needed to be relatively simple and easily accomplished in the field. All this had to be done without shutting in the well or using chokes, since that would also cause breaking down the upper zone and result in an underground blowout. This paper describes the development of the required methodology and its successful field application in South Texas. This same technique can readily be applied elsewhere. Introduction The operating company has been aggressively pursuing drilling cost reduction in the Lobo Field of South Texas. A leading technology being used in these efforts has been Casing While Drilling (CWD). In an effort to further reduce costs, Underbalanced Drilling (UBD) was identified as a possible way to eliminate an intermediate casing string by drilling formations of differing formation pressure and fracture gradient simultaneously over a long openhole interval. By using CWD, the UBD operation was greatly simplified since trips were no longer necessary. While drilling like this could obviously be done, the question remained of how to cement a well that could not be killed or balanced conventionally or even shut in without severe losses and downhole crossflow. Historically, wells in the Lobo field follow a well plan commonly used throughout the world. Surface casing is set at a depth sufficient to protect usable fresh water, an intermediate string set near the top of the producing formation isolates the weak shallow zones (Queen City in this case) from the elevated mud weights required to balance the pay zone. The mud weight is then increased, the well drilled to TD, and production casing set. Things are further complicated in this field by the presence of a relatively weak zone, designated the "A" marker, 500-1500 ft above the target zone. The strength of both the Queen City and "A" marker vary throughout the field and good breakdown pressure data was unavailable for the "A" marker. Calculations made using a hydraulic simulator (HUBS) indicated that it should be possible to successfully balance the well dynamically at TD without losing returns in the shallow formations and to cement under those conditions by adjusting pump rate during the cement job to maintain a constant bottomhole pressure sufficient to prevent gas influx and cement contamination. Some adjustments in wellbore geometry would facilitate this. Since there were some uncertainties in how the formations would actually behave under these conditions, a three-well test program was selected and begun in 2003 to progressively test this new technique under field conditions.