A new method of gravel-packing a well is presented emphasizing the removal of installed damage, the prevention of new damage by avoiding formation fluid loss, and the use of an inexpensive and safe frac-pack tool.
In unconsolidated formations, even for low overbalance, the rate of fluid loss is considerably high. The fluid loss generally causes damage which, in many cases, may require a workover job to be removed If the operator allows fluid loss to the formation, some problems such as water-in-oil emulsion, shale swelling, water block and salt precipitation may occur.
This work shows a new design in gravel-packing (GP) operation, which allows setting a sump packer, perforating, flowing the well for damage removal and gravel-packing in just one run, thus permitting to work in an underbalance way. The new design also presents a new fracpack tool which is simpler and less expensive than conventional tools.
The new design was conceived to solve critical problems in gravel packing in Dourado Field, in the northeastern Brazilian Continental Shelf. The use of the traditional methods resulted in very poor well performance. The lack of compatibility between rock and water base fluid was considered to be the main reason for the problem. The application of the new gravel-packing method turned-out to be the key for the successful development of the field.
Recent papers concerning GP stress the need to avoid completion fluid interaction.
A high-density pre-pack followed by auto-injection of salt cushion to seal the perforations before GP and the utilization of a ceramic flapper-valve in the top-inside liner after GP are nowadays useful in solving this problem.
These methods do not avoid but just reduce fluid-rock contact. And they do not allow in many cases the use of a total rock-compatible fluid, as diesel oil, for instance, due to the high formation pressure gradient and the risk of overbalance trips.
A damage is very likely to be installed during previous operations, main job and subsequent trips, as follows:
Before GP: the main causes of damage are perforation problems (tunnel collapse and crushed zone) and mud, completion fluid and/or cement filtrate losses.
During GP installation: the main causes of damage are:
inappropriate completion fluids, such as polymers and non-filtered salt-water,
formation sand versus gravel mixing,
collapsed or formation-sand filled perforation tunnels,
formation loss of clean completion fluid and
loose or non-continuous pack.
After GP: the main cause of damage is completion fluid loss.
Some considerations about each of these problems are as follows:
Loose, or non continuous packing - the voids or loose packing are causes of a non-stoppage-sand packing or poor well performance, due to gravel-sand mixing.
Formation-sand tunnel filling and crushed-zone - both are reasons for poor well performance due to high friction pressure loss.
Water-base completion fluids - all of them are potentially damaging. Emulsion, water block, sludge, chemical precipitation, swelling and migration of shales, polymers and salt deposition are some of the problems that can result from their use.
Some recommendations for a successful gravel-pack would be:
Related with prevention or removal of damage:
Before GP - let the well flow in a controlled way.