Effect of Sand invasion on Oil Well Production:

The probability/tendency of producing hydrocarbon from reservoirs to the surface production facilities in a safe and economical way without the inclusion of sand is very low from a typical Niger Delta field due to the unconsolidated nature of the formation. To maximize profit in this region, it is crucial to identify all the possible problems that affect the productivities of the wells drilled into the reservoir and sand production is one of these problems plaguing the production aspect of the oil industry. Moreso, geological formations that are shallow with little or no natural adherence to hold the individual sand grains together are normally prone to sand production but in some areas, sand problems may be encountered in high depths (Adams, 1986). To effectively control the invasion of sand, we need to have technology to estimate accurately the initiation conditions, predict the sand influx rate and the volume of sand production to prevent its effect on surface and subsurface production facilities which directly impact on the oil well productivity. Oluyemi and Oyeneyin (2010) stated that the economic, operational and safety implications of sand failures require real time efficient sand management. Since the effect of sand on facilities is costly, Coberly (1941) noted that expenditures of this magnitude obviously have a significant impact on profits. In spite of these costs, effective sand-control practices have yielded oil and gas from wells that otherwise would have been shut- in. The rationale behind sand production is that; as soon as the well begins production of fluids, depending on the formation type and other factors, a point is reached when sand mobilization sets in. Sand production in oil and gas wells can occur if fluid flow exceeds a certain point governed by factors such as consistency of the reservoir rock, stress state and the type of completion used around the well, this happens when the wellbore pressure is lower than reservoir pressure, because drag forces are applied to the formation sand sequel to fluid production. The sand production takes place if the sand grains around the cavity is disaggregated and as the volume of sand dislodged is deposited and accumulated on production equipment continuously, cleaning will be required to allow for efficient production of the well. To restore production, the well must be shut-in, the surface equipment opened, and the sand must be manually removed. In addition to the clean out cost, the cost of the deferred production must be considered

The design of the surface separator is to handle liquid and not sand production which essentially has no economic value with adverse effects on well productivity and equipment. This is one of the major challenges that is facing the petroleum industry especially the production operations in many oil fields in Nigeria whose formation is unconsolidated and some other countries such as US gulf coast, California, Indonesia, Trinidad, Venezuela, and Libya. If a separator is partially filled with sand, the capacity of the separator to handle oil, gas and water is reduced. For instance, in a two phase separator (oil/water), for every 5000 barrel of liquid per day on average is accompany by a corresponding 0.03 pptb of sand per day. It implies that at the end of say 6 months the separator must have accumulated about 5.472 pptb of sand and if this continues, an appreciable volume of the separator designed to handle liquid must have been partially filled with sand. In Garon field used as a case study, well X17 was routed into Test separator with the Test separator pressure set point at 20Bar. The well was routed out of Test separator after three (3) days. The well was tested for 72Hrs at 65/64” choke size. On the second day, test separator man way was opened in other to ascertain the quantity of sand produced from the well for 72Hrs. results are shown in Figures 2-6.

II.CAUSES OF SAND PRODUCTION

The causes of sand production can occur naturally as a result of the unconsolidated nature of the formation or by the activities on the well imposed by humans because there are only two ways to disturb a reservoir which are via production of fluid from it or injection of some fluid into it. When this happens, it causes agitation of the formation loose fines to disintegrate from the rock grains and as such leads to sand production along with hydrocarbon fluid. As stated by Anderson et al, (1986) that mechanical rock failure can be caused by any or more inherent rock strength, naturally existing earth stresses and additional stress cause by drilling or production. In totally unconsolidated formations, sand production may be triggered during the first flow of the formation fluid due to drag from the fluid or gas turbulence which detaches sand grains and carries them to the perforation. In the case of the unconsolidated formation, sanding can start due to changes in production rate, water breakthrough, change in gas/liquid ratio etc. Therefore, these causes can be lead to several problems during the life time of the wells drilled in a particular reservoir which is a major production engineering problem.

These can lead to one or more of the following complications:

· Formation damage or collapse by the flowing sand grains

· Wellbore instability

· Casing collapse

· Impairment or failure of down hole and surface equipment

· Lost production time due to shut in of the well to change damage equipment or clean the sand filled wellbore.

· Work-overtime and expense to service the well and production equipment

· Coiled tubing cost and possible complications

· Cost of separating sand from the produced fluid

· Environmental problems in the disposal of the produced dirty sand Fjaer, et al (1992)

· Cost of sand disposal and control measures at the pay zone

III. Collapse of the formation as result of sanding

Large volumes of sand may be carried out of the formation with produced fluid. If the rate of sand production is great enough and continues for a sufficient period of time, an empty area or void will develop behind the casing that will continue to grow larger as more sand is produced. When the void becomes large enough, the overlying shale or formation sand above the void may collapse into the void due to a lack of material to provide support. When this occurs, the sand grains rearrange themselves to create a lower permeability than what originally existed. This will be especially true for formation sand with a high clay content or wide range of grain sizes. For formation sand with a narrow grain size distribution and/or very little clay, the rearrangement of formation sand will cause a change in permeability that may be less obvious. In the case of overlying shale collapsing, complete loss of productivity is probable and in other cases, continued long term production of formation sand will usually decrease the well’s productivity and ultimate recovery. The collapse of the formation is particularly important if the formation material fills or partially fills the perforation tunnels. Even a small amount of formation material filling the perforation tunnels will lead to a significant increase in pressure drop across the formation near the well bore for a given flow rate