Isochronal testing is commonly used to evaluate the performance of gas wells. This paper proposes a new technique to estimate the value of turbulence coefficient based on isochronal tests. The proposed method is easy to apply and evaluate. Further, the method also provides a value of bg under stabilized conditions which can be used to predict the performance of gas wells under stabilized conditions.
The proposed method is validated using field data under a variety of operating conditions. The values of turbulence coefficient based on the field data can differ significantly compared to the literature correlations. This further shows the importance of obtaining appropriate reservoir parameters based on the field rather than the lab data.
The use of isochronal or modified isochronal testing is well established in the gas industry. These tests are common for gas wells which take a long time to reach a stabilized rate. A common example would be a low permeability, fractured reservoir. Instead of testing these wells till a stabilized rate is reached, the wells are tested for a fixed period of time and the bottom hole pressure is measured. For isochronal testing, the well is then shut in till it reaches a stabilized pressure and the procedure is repeated for different rate. For modified isochronal testing, the well is shut in for a fixed period of time, and the shut in pressure is measured at the end of that period. The procedure is then repeated at other rates.
By repeating this procedure for different time intervals, we can gather information about rate versus pressure drop in the formation for these time intervals. Ultimately, using this information, our goal is to establish an appropriate rate versus pressure drop relationship under stabilized conditions.
Two procedures are commonly used to establish the equation for rate versus pressure drop. An empirical method states that,
where qg is the gas rate, C and n are constants and p and pwf are average pressure and bottom hole pressure respectively. We can write a simpler equation in terms of pseudo-real pressures as,
where m(p) and m(pwf) are pseudo-real average pressure and pseudo-real bottom hole pressure respectively.
Under transient conditions, the value of C is not constant. Instead, we can write Eq. 2 as,
where c(t) represents a term which is a function of isochronal interval t. In literature, methods are proposed to estimate the value of C corresponding to stabilized rate based on the transient state information −C(t). See e.g., Hinchman et al. In that paper, Hinchman et al. propose that be plotted as a function of log t, and the line be extrapolated till t is equal to the time it takes to reach stabilized state period. P. 223^