Several technical papers discussed the importance of well completion designs which allow fines (particles below 40 microns) to be produced out of the well in order to avoid plugging and loss of well productivity. SPE paper 14813 discussed the concept in particular, which further confirms that formation particles below 40 microns must be produced out of the producer in order to avoid plugging of down-hole sand screens and the associated loss of well productivity. Screen plugging is linked to screen slot or pore opening. Lau and Davies's (SPE 38638) introduced the 1/7th rule. This rule states that screen plugging is linked to screen slot or pore opening in accordance to the 1/7th rule: solids with D50 larger than 1/7th of the screen slot opening will plug the screen.
Other SPE papers (SPE 56813, 36419, 71673) also supported the concept that when properly engineered, fines are allowed to be produced through wire-wrapped screens, and plugging is therefore prevented. Based on numerous lab experiments and field observations, it can be concluded that the following observations and basic engineering principals are true:
» Screen construction & slot sizing must ensure that fines
(2) can be produced unhindered through the openings of a screen.
» Remaining and non-produced sand
(3) will form a bridge on the screen surface, and creates a natural sand pack with higher porosity and permeability than the formation
(1).
» Evidence shows that the installation of a Stand Alone Screens in an open hole will allow the formation to de-stress. Formation de-stressing occurs only in a screen-only sand control completion, but never in a gravel-packed completion. In a screen-only completion the near well-bore formation is allowed to expand during the process of depletion and compaction. The increased overburden caused by compaction leads to increased stress in the near well-bore area. This often results in the collapse of sand grains, and the process continues throughout the entire production life of the well. With the available annulus (between a screen and the borehole) the formation is allowed to expand, and with a properly engineered wire-wrapped screen, fines are allowed to be produced out of the wellbore. The expansion of the near wellbore formation leads to the increases of both porosity and permeability of the formation. Under such a condition, formation porosity within the affected vicinity shall normally increase 30-40 %, and formation permeability in the area shall increase 3 fold.
When using gravel to fill an annulus in a gravel-packed completion, expansion and de-stressing cannot happen. Therefore, the increased near wellbore stress shall lead to the loss of formation permeability and a declining PI over the production period. If the screen (in a screen only completion) becomes plugged by fines, the increase of the corresponding skin will mask or cancel the positive effects of near well-bore expansion. Only in a properly engineered completion (which allows fines to be produced) can such positive rock mechanic effects of formation de-stressing be harvested. As documented in numerous technical papers, the screens which are the least susceptible to plugging are those called single wire-wrapped screens. Depth filter screens (pre packed wire wrap screens, mesh screens and porous metal membrane screens) are much more prone to plugging, and must be applied with caution in any open-hole completion. |
