Because of frictional losses in flow, there is a pressure drop along a horizontal wellbore. Should this pressure drop be significant relative to drawdown, the pressure far down hole could nearly equal the pressure in the reservoir, rendering part of the well unproductive. Friction can thus reduce productivity. Systems likely to suffer from this effect include those with low drawdowns in highly-permeable formations, those with long horizontal sections, and those with wells of small diameter. This paper provides easy-to-use guidelines that indicate whether wellbore friction can be neglected.
With a model of steady, single-phase flow and with particular values of the relevant physical parameters, one can compute the length of a well that has a 10% loss of productivity because of friction. Computation of this length in thousands of hypothetical cases leads to simple criteria that indicate whether wellbore friction matters in a particular well. These guidelines are based on well length, production rate, hole diameter, and roughness of the wellbore. A reliable indicator is the ratio of wellbore pressure drop to drawdown at the producing end; when this ratio exceeds 10%, friction is apt to reduce productivity by 10% or more. In general, immune are gas wells that produce less than 2 MMscf/D and oil wells that produce less than 1500 STB/D.
Horizontal wells are becoming an established method for the recovery of oil and gas. In reservoirs where these fluids occupy strata that are horizontal or nearly so, a horizontal well offers greater contact area with a productive layer than does a vertical well. Larger contact areas allow lower drawdowns to recover more oil or gas, reducing the potential both for gas coning and for water coning. In addition to reducing coning tendencies, horizontal wells can enhance production by contacting several naturally-occurring, vertical fractures.
Economics aside, these facts imply that a horizontal well should be as long as possible, and drilling technology now allows wells with lengths of several thousands of feet. There is, however, a factor that can possibly limit the useful length of a horizontal well: frictional losses in the wellbore. In cases with long wells or high flow rates, the pressure drop in the wellbore might be comparable to the drawdown at the producing end of the well. A portion of the well down hole would then be unproductive because the local drawdown there is small or even zero. The expense of drilling and completing that portion of the well would then be wasted.
Researchers have sought to identify the situations in which loss of drawdown along a horizontal well reduces productivity and to quantify the effects of friction in a horizontal wellbore. Several theoretical efforts have been reported.