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Much natural gas and oil cannot be recovered from tight low-pressure reservoirs because of formation damage from drilling fluids. The severity of the drilling fluid damage depends on the quantity and properties of the fluid filtrate. Two means of minimizing formation damage during drilling can be considered:

  1. controlling the properties of the drilling fluid by using nondamaging drilling fluids, and

  2. minimizing the amount of liquid filtration into the reservoir.

However, developing and using non-damaging drilling fluids, if possible, is expensive. The second means is, therefore, considered as a better option. Since the cause of liquid filtration into the reservoir is the overbalance pressure against the reservoir, this paper concentrates on the use of the aerated mud to minimize this overbalance pressure.

It is highly desired to maintain an optimum combination of air and mud flow rates so that the wellbore pressure is as close to the reservoir pore pressure as possible while the wellbore is kept stable and drilling cuttings are effectively transported to the surface. However, it remains unclear to drilling operators as to what constitutes the "optimum combination" of air and mud rates. This paper addresses the analysis of wellbore pressure and carrying capacity of an aerated mud, determination of the optimum combination of air and mud rates, and estimation of filtrate invasion into the reservoir when aerated mud is utilized as a drilling fluid.

Figure 1 provides flowing bottom hole pressure information for aerated mud drilling for different combinations of mud flow rate and air injection rate. The cuttings carrying capacity of the aerated mud is given in Fig. 2. The "S-shaped" behavior of curves in Fig. 2 is explained in the paper. Based on computer simulation, a series of wellbore pressure and carrying capacity charts are generated and provided for drilling and reservoir engineers in the paper. A simple method is developed for determination of the optimum combination of air and mud rates using these charts. Figure 3 shows the relationship between the over-balance pressure and radius of the filtrate invasion into the reservoir. The curves in Fig. 3 are plotted based on Outmans' filtration model. Employing Fig. 3, it is demonstrated that the use of aerated mud drilling with the optimum combination of mud and air flow rates will result in less formation damage.

For the reservoir and drilling data given in Table 1, if water is used as drilling fluid to drill the well, the minimum bottom flowing pressure is calculated to be 4,400 psi, which is 905 psi over the reservoir pressure. According to Fig. 3, this overbalance pressure would cause 5.6 feet of filtrate invasion around the wellbore! If aerated water is used as the drilling fluid, the optimum combination of water flow rate and air injection rate can be determined from Fig. 1 and Fig. 2 as 260 gallon per minute of water with 600 cubic foot per minute of air. If this optimum combination of liquid and air flow rates is used in the aerated mud drilling, the reservoir pore pressure is slightly overbalanced and, based on Fig. 3, the filtrate invasion radius will be only about 1.4 feet.

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