This paper presents the case studies of the reservoir performance while under-balanced drilling. The increase of the length of the well bore provides a unique transient effect that, with an appropriate mathematical model, some reservoir parameters can be obtained. The mathematical model used for this study is the discrete flux element method1. Applying this mathematical model, Fracmaster Ltd. analyzed the bottom hole pressure data from two coiled tubing under-balanced drilling jobs. Unlike the well testing of well bores with constant well length, for this method, due to the complex transient effect of draw down and increase of the physical length of the well, the calculated reservoir parameters are almost unique. Due to the increase of the wellbore length, there is almost a continuous pulse reaching the upper and lower boundaries. Thus the solution is very sensitive to the reservoir thickness.
Underbalanced drilling is a growing technology that has certain advantages compared to overbalanced drilling. In underbalanced drilling, the wellbore pressure inside the annulus is to be kept less than that of the reservoir pressure. This allows reservoir influx into the wellbore while drilling to prevent the invasion of the drill cuttings into the reservoir matrix. To obtain the underbalanced condition one has to lighten the weight of the fluid inside the annulus to decrease the hydrostatic pressure. The usual method is by injection of a neutral gas such as nitrogen through tubing. The amount of the reservoir influx is a function of the underbalanced degree (pressure drop at the formation) as well as the well bore length (as the drilling proceeds the length of the well bore increases). The increase in the producing length provides a unique transient effect that can be used to analyze the reservoir performance and to estimate some reservoir parameters such as reservoir thickness, permeability, and the trend of the increase or decrease of the productivity along the wellbore.
In well testing of horizontal wells with constant length, the effect of the reservoir thickness may be observed at short times but most of the time it is masked by the wellbore storage. However, in underbalanced drilling, increase in the wellbore length creates a continuous disturbance in the drainage zone. These disturbances, in the form of pulses, hit the upper and lower boundaries. Therefore, the solution and the reservoir performance are very sensitive to the reservoir height and the reservoir diffusivity.
For well testing of horizontal wells with constant length, using several different sets of the parameters, one can satisfy the reservoir performance. For underbalanced drilling there is one unique set of the parameters.
Several factors affect the influx rate. The most important factors are rate of penetration (ROP), or drilling velocity, pressure drop at the wellbore while drilling or amount of under-balance, wiper trip periods, shut-ins and flow tests.
The influx rate at any time is a function of the drilling activity history. The improvement or deterioration of the reservoir quality (productivity) along the wellbore could not be inferred from the influx rate at a given time without consideration of the complete drilling history.
