An Improved Procedure for Handling a Threatened Blowout
Authors: Schurman G.A. 
Published: December 2007

A graphical method is described for calculating quickly and conveniently the maximum casing pressure that should be observed when circulating up a gas kick; the complete schedule of casing pressure vs the volume of new mud pumped for circulating up either a gas or water kick; and the mud gain required while circulating up a gas kick to maintain a constant bottom-hole pressure. The procedure differs from previous methods for calculating pressures in that the maximum casing pressure (with gas) is determined graphically using a generalized pressure-volume chart. The casing pressure vs volume new mud pumped schedule is determined on a standard chart having coordinates of pressure and barrels of new mud pumped. The generalized pressure volume chart for a gas kick is a transparency and is used in conjunction with the pressure-mud graph to provide rapid, graphical determination of the maximum static casing pressure and the maximum volume of mud gain. All calculations can be made with a slide rule. The basis for the method, construction of the universal pressure volume chart and sample worksheets are described. An example of a field application also is included. The procedure has been used numerous times under field conditions and has given excellent results. Experience shows that the complete calculation can be made at the rig site in about one half hour.

Introduction

Numerous articles have been written about blowouts, threatened blowouts and practices for avoiding trouble. The early papers were qualitative, discussing rig practices, crew training and the importance of testing blowout preventers (BOP's). These papers were aimed at avoiding a kick in the first place, and contained little advice on what to do with pressure on the preventers. The papers by Bell and O'Brien et al. mark the beginning of an engineering approach to well control. The procedure published by O'Brien et al., often called the drill pipe pressure method, gives the operator a way of determining in advance how he should proceed to maintain a constant bottom-hole pressure during the killing process. Keeping the bottom-hole pressure constant is the correct way to handle the well; it minimizes the risk of breaking the well down, causing lost circulation, and avoids additional flow into the well. In practice, the drill pipe pressure method has two limitations that we consider serious. First, the procedure does not allow the operator to calculate maximum expected casing pressure before he starts to circulate the kick up. Second, keeping a constant bottom-hole pressure by adjusting the drill pipe pressure can be tricky - particularly if the pressures are high and if the bit has small jets. The next important development in well control was published by Records et al. They also apply the principles of hydrostatic to the fluids in the wellbore and additionally include a calculation procedure for handling the expansion of the gas as it is circulated up. Consequently, the operator can calculate in advance the maximum expected casing pressure and a schedule of the variation of the casing pressure during the killing process. In principle, this approach is accurate and should give the operator the maximum amount of information. In practice, the method has been found to be difficult to use. The step-by-step calculation is tedious and slow and the chances of making arithmetic errors are high. The procedure presented here is an attempt to maintain the quantitative approach to well control, as presented by the above authors, but to simplify the calculation procedure. Simplicity is obtained by using a graphical procedure and a universal gas expansion chart. The procedure is approximate, but the accuracy is considered to be consistent with the accuracy of other measurements made at the rig.

Theory

After a well has kicked or been swabbed some volume of contaminant has entered the well and the situation that must be handled is illustrated in Fig. 1a. The well is represented as a U tube; the thin side represents the drill pipe and the thick side the annulus. The formation pressure Pws, the drill pipe pressure Pdp and the casing pressure Pcs are illustrated in the figure. To kill the well correctly, new mud just heavy enough to balance Pws must be pumped into the well and the gas bubble must be circulated out. One way of keeping the bottom-hole pressure constant and equal to Pws is to adjust Pcs so the sum of Pcs and the hydrostatic head of all of the fluids in the annulus equals Pws. Compressibility of the gas is the factor that makes the casing pressure calculation difficult.

JPT

P. 437ˆ