Flow Capabilities of Horizontal Wells

Abstract

"Although horizontal wells have the potential for increased productivity because of their lengths, other factors... may offset... the length advantage." Advances in drilling technology have focused the oil industry's attention on the importance and viability of horizontal wells. Reasons for drilling horizontal wells areto increase productivity,to intersect and drain vertical-fracture networks more effectively,to reduce coning, andto increase sweep efficiency. Our consideration, here, is horizontal-well productivity, which can be estimated with productivity, which can be estimated with the use of recently published formulas. The inset in Fig. 1 indicates the physical model of a horizontal well producing at a constant rate (shown with a centrally located vertical well for comparison) located in a box-shaped drainage volume with closed faces. The impermeable faces could be the result of faults, pinchouts, or symmetry conditions caused by the presence of other wells in the field; thus, they portray realistic boundary conditions on the flow detail. Because we are interested in the potential productivity of the well for large times, we productivity of the well for large times, we present only the pseudosteady-state flow present only the pseudosteady-state flow behavior. We represent this PI by J, which is the flow in stock-tank barrels per unit pressure drawdown. pressure drawdown. Given the same box-shaped drainage volume, under what conditions will the PI of a horizontal well exceed that of a vertical well? Before answering this question, we compare the variables that affect the productivity of both horizontal and vertical productivity of both horizontal and vertical wells. Generally, the horizontal well is longer than the vertical well. However, the magnitudes of three variables-the crosssectional areas perpendicular to the well direction, geometrical mean permeabilities of those cross sections, and partial penetration ratiosare much less for the penetration ratiosare much less for the horizontal well than those for the vertical well. The productivities of both wells are directly proportional to their lengths and mean permeabilities and are adversely affected by permeabilities and are adversely affected by smaller partial penetration ratios and cross-sectional areas. Given these statements, it is clear that well length, L, alone, contributes to the increase of a horizontal well's productivity, while the other items favor the productivity, while the other items favor the vertical well in most practical cases. In addition, if a sizable vertical-fracture network is present, leading to large magnitudes of vertical permeability, kz, then the horizontal well will gain substantially in its mean permeability and, thus, in its productivity. productivity. Fig. 1 compares the productivities for horizontal and vertical wells. Productivity of the horizontal well, J H exceeds that of the vertical well, J V, above the dotted line. Beneath this line, the vertical well performs better. On the dotted line, production for both wells is the same. Note the significant influence exerted by partial penetration, as well as by vertical-permeability values, in changing the productivity. P. 914