Abstract
Sanding is a common phenomena during gas production in S gas reservoir due to the loosen sandstone formation, furthermore, the rock inner cohesion will be weaken by
the water produced from interbed water and edge water, as the result, sand production in-situ is enhanced. Drawdown controlling is the reasonable strategy to sand control. To calculate the critical drawdown, a new model, which is based on the theory and lab testing data analysis to the rock composition, physical and chemical mechanism,
cohesion weaken by watering, is developed in this paper. In this model, rock cohesion is considered as a function of mud content and changing with water saturation, which is
a typical performance in the loose sand gas reservoir. It is more reasonable and reliable to forecast the sanding critical drawdown by this new model compare with field sanding monitoring data.

1 Introduction
Sanding usually exists in S gas reservoir which is characterized with loose sandstone. Sanding phenomena which is observed in field and in laboratory testing is classified as 3 types as rock cohesion:
        1)     Weak cohesion: sanding is triggered at the beginning of production, or at the second well opening. The main reason of sanding is the tensile failure during the formation shearing and well clean up.
        2)     Medium cohesion: there is no sanding at all at the beginning of production. Sanding is happened only when the water is produced since sand grain is washed off from the skeleton of rock and the cohesion is reduced accordingly.
        3)     Strong cohesion: sanding will happen due to shear failure even to the stronger rock if the effective stress exceeds its compressive strength. Rock with higher cohesion will with less sanding opportunity.
Some technical measurements have been applied in sand controlling. However, these measurements would increase the cost of well completion and boost the risk of sanding out-of-control, and could restrict well productivity, either.  Drawdown controlling is the reasonable sand preventing strategy with low cost and low risk.
Physical and chemical mechanism of sanding in loosen sandstone is analysis and the sanding critical drawdown is calculated to direct the production rate allocation.

2 Sanding Mechanism Analysis
2.1 Mineral Composition
Clay compositional analysis to the rock sample which is taken from well S3-15 shows that the main clay mineral is illite, with average content 51%, second is the mixture of
illite and magnalite, with average content 21%, chlorite is 19% and with some kaolinite. The average clay content is 47% with maximum 85% and minimum 17%.
Swelling and dispersing due to water sucking in the illite will increase the flow rate sensitive and water sensitive. The mixture of illite and magnalite is unstable and easy to be separated.
Rock grain is easy to be transferred and lead to the flow rate sensitive due to the weak bonding force of crystal lattice in Kaolinite.
Watering will reduce the rock cohesion and enhance the sand production due to this special mineral composition in loosen sandstone gas reservoir, as a result, this kind
reservoir is characterized with lower sanding critical flow velocity. The effect of sanding and flow rate sensitivity must be taken into account during the gas wells production management and their dynamic performance forecasting.
2.2 Sanding Phenomena in Gas Reservoir
There are two sanding sources: skeleton sand and filling sand. Quartz and feldspar is the main composition of skeleton sand, which looks like bigger sand grain. Clay
mineral and particulate is the main composition of filling sand, which is observed as fine grains around the skeleton sand.