The current interpretation of gravel pack productivity decline seems to focus on productivity decline seems to focus on dispersion and plugging with formation fines, especially after acidizing. This paper presents an alternate explanation. paper presents an alternate explanation. Gravel may be flushed from perforations during acidizing into acidized formation voids, gravel may move into acidized gravel pack voids in the screen-casing annulus during production start-up, or tunnel voids may have existed prior to acidizing as a result of poor gravel placement during gravel pack execution. placement during gravel pack execution. Production start-up rate controls how Production start-up rate controls how formation sand enters partially filled voids after acidizing. A controlled production rate allows formation sand to production rate allows formation sand to enter the tunnel above the gravel so that a more conductive parallel alignment of sand and gravel in the tunnel is created.
Gravel packing technology has improved dramatically in the last 15 years. This focus on improvement was generated by poor well performance compared to formation potential after proration controls were potential after proration controls were removed in times of oil shortage. Early models focused attention on the pressure drop caused by flow through gravel packed tunnels in inside-casing gravel packs and pointed out the effects of turbulent flow pointed out the effects of turbulent flow (or visco inertial flow) at high velocities through these gravel packed tunnels (refs. 1–4). Gelled water systems were developed and studied for more effective placement of gravel. Improvement in gravel sizing and gravel quality control led to improved performance, especially after sufficient performance, especially after sufficient perforated area was created by using larger perforated area was created by using larger diameter perforations. Nodal analysis focused attention on the selection of shot density and hole diameter. On site quality control also enhanced productivity. Even with improved perforation cleaning, formation damage was still encountered after placement of gravel packs.
During all this development damaged gravel packs were acidized to improve performance. packs were acidized to improve performance. Often wells responded favorably and then declined rather quickly after acidizing. Other wells responded and maintained good performance through use of special acid performance through use of special acid systems and controlled production rate. Yet sometimes different wells responded well to regular acid systems and poorly to special acid systems. Acidized well performance improved gradually with better job planning, execution, quality control and planning, execution, quality control and improved acid placement. Because of the variability of acid response, a study was undertaken to better understand the response of inside casing gravel packs to acid.
Wells were examined to see how damage was caused and why acid did or did not work. One finding was that after good quality gravel pack placement, extreme damage still occurred that could not be explained satisfactorily by damage to the gravel in the tunnels. This damage could reasonably be modeled by damage in the formation sand adjacent to the gravel packed tunnels. A model incorporating this packed tunnels. A model incorporating this effect assumed hemispherical flow through the formation to the gravel packed tunnel entrance.