In deviated wellbores, the relationship between friction pressure and hydrostatic pressure differs from that in vertical wellbores. The change in this relationship increases the need for accurate slurry friction pressure modeling. Because of deviation, incremental slurry friction pressure changes are more significant than incremental changes in hydrostatic pressures. Real-time calculation of an accurate bottomhole treating pressure in deviated wellbores is more difficult to achieve, than in vertical wells, because of the lack of accurate slurry friction models.
'Abnormal' surface treating pressures, which are caused by this change in relationship, were recorded while fracturing deviated wells in the Prudhoe Bay Field in Alaska. The significance of this relationship is presented along with observed slurry friction pressure data obtained through the use of bottomhole pressure recorders. A methodology and mathematical model for more accurate calculation of slurry friction pressure is presented for these conditions.
While performing fracturing treatments in vertical wells with the tubular and treatment conditions presented here, the surface treating pressure will normally decrease when proppant is added and proppant concentrations increase. 'Abnormal' or increasing surface treating pressures have been recorded while fracturing highly deviated wells. Misinterpretation of these surface pressure increases can cause erroneous decisions to be made during the fracturing treatment.
Figure 1 is a hypothetical plot of surface treating pressure, slurry rate, and proppant concentration versus time for a frac job performed in a near vertical well. This example shows a continual decrease in surface treating pressure with proppant addition. Figure 2 is a hypothetical plot of the same parameters on a well that has a high angle of deviation from vertical. Note that at first, the surface treating pressure decreases when proppant is added but then begins to increase as the proppant concentration increases.
The hydrostatic and tubular friction pressures must be accurately calculated during the treatment in order to obtain the net bottomhole treating pressure for proper analysis. Hydrostatic pressures are fairly simple to derive if the slurry density is monitored at the surface. The tubular friction pressure must be calculated based on a friction pressure model that is incorporated into the real-time computer monitoring package. This parameter is much more difficult to predict because of the variables involved. Because of the higher significance of friction pressure in highly deviated wells, proper slurry friction pressure modeling is even more critical.
To properly understand this pressure phenomena and its significance, one must first have a thorough understanding of surface treating pressure and its components.
The fracturing surface treating pressure is a function of four basic pressure components: hydrostatic head, pipe friction pressure, perforation friction pressure, and the formation fracturing pressure or bottomhole treating pressure. Their relationship is expressed in the following equation: