Well losses are associated primarily with turbulent flow as velocities increase toward the well bore, and may be particularly important in a formation where most or all flow is through fractures...the effects are manifested primarily by head losses in and immediately around the well bore, and will be superimposed on the effect of formation losses. Formation losses are those head losses, generally assumed to be associated by laminar flow, as water flows through the formation toward the pumping well. The total drawdown in the pumped well should be the sum of these two factors, and analyses of step drawdown data should provide you with some estimate of the relative significance of well losses and formation losses (similar to the idea of well efficiency). Two other approaches to assessing this are a) analyses of recovery data for the pumping well (well losses should be minimized), or b) if observation well data is available, you may be able to estimate the theoretical drawdown at the pumping well without well losses (ie an estimate of head losses associated with just the formation).

Thank you Sir. I got your point. But what I am essentially interested to know is that - well loss practically is because of constructional attributes of the well - constructed in semi-consolidated and unconsolidated formations. In consolidated formation, the fracture zones are generally in the uncased part and as such can be assumed to be formational attribute only. Now in such case if I get the well loss value(graphically using BQ + CQ^2 formula) - what should I interpret it to be.

Anirvan - as best I know, regardless of the well construction details, the well loss portion of overall head losses is associated with turbulent flow in the immediate vicinity of the well bore (I note that even with a cased well, it is only the uncased portion or screened portion that has a bearing on this... it is a function of water velocities inceasing as radial flow converges on the well bore...thus this component of drawdown relates to conditions in this specific zone, and is added on top of the drawdown due to head losses associated with flow through the formation. The well loss value should therefore tell you how much of the total drawdown is attributable the specifics of the well bore, which depending on your goal, may be able to be manipulated (it will be highly sensitive to pumping rate ...approximately an exponential relationship). If your goal is to reduce drawdown, lower pumping rates are in order. If you wished to increase well yield, you could try enlarging the borehole diameter which may result in somewhat lower turbulent head losses as the water enters the well.

Anirvan,

To follow-up on George's outstanding response...I would recommend that you review the description for well efficiency (Wu) found in both Fletcher Driscoll's "Groundwater and Wells" and C. W. Fetter's "Applied Hydrogeology" (sorry I can't be more specific -- but I don't have my references in the office yet).

I have always thought of fracture flow as analogous to an open channel problem (e.g. I need to account for the head loss related to the roughness of the channel itself -- Manning Equation). If I were to use monitoring wells, I would need to account for the dominant joint and fracture system of the locale, in that the radial influence of an induced stress would be expected to be asymmetrical along the axis of the dominant fracture pattern of the host rock and along the hydraulic gradient.

Since you've identified three distinct rock-types in your description...I'd imagine that you'll end up having at least four distinct boundary conditions to consider (three from the differences in the rock units, and some basal confining consideration. Based upon your basal confining boundary -- the vertical gradient can be either negligible or substantial (no surprise there I imagine).

Hope this is of some assistance...sounds interesting.

All my best...

Greg

Anirvan, I think George's analysis is sound. "Formation loss" is an alternate way of referring to radial-flow drawdown in the aquifer that adheres to the Theis solution at the interface between the "Theissian" portion of the aquifer and the well radius (wherever that may be). At some sufficiently low pumping rate, formation loss should approximately equal the total drawdown in the step-drawdown segment. As pumping rate is increased, more deviation from the Theissian drawdown is observed, presumably due to greater turbulence. In a screened or slotted well in granular aquifer, these well losses are also termed entrance losses, referring to the flow convergence associated with water moving from the aquifer into slots, screen opinings, etc. Well losses may also be increased by fouling due to sine-grained sediment outside the screen not being cleaned out by development. In a fractured aquifer without casing, the same principles apply but the turbulence may be associated with the fracture pattern intersecting the well. In general, the lower the area of continuous fractures intersecting the well bore, the higher the well losses will be at any specific discharge. But fracture roughness etc could be a factor as well as the degree of homogeneity of fractures outside the well bore. You are correct that the concenpt is more applicable in granular formations.

Note that the exponent of the turbulent loss term may be fit empirically in addition to using the "2" value in the equation you cite, which is based on compliance with turbulent head losses in the Bernoulli equation. The basis of interpreting a step test is to separate the component that is linearly related to Q (per the Theis solution) from the component that is related to a higher power of Q suggesting turbulence in the well bore. Recall that karst aquifers with conduits, for example, are known to display turbulence and for that reason, Darcy's Law and analytical solutions based on it are frequently inapplicable, particularly near wells pumping at high rates....

You probably have nonlinear flow around the extraction zone. This has the same effect of well loses in casings.

Thank you every one for your quick and kind response. It was indeed eye opening. What I got from the above knowledgebase - please correct me ruthlessly, if I am approaching wrongly. Here we are basically concerned with the well hydraulics mainly. The system that we are dealing with is a dual or multiple porosity system, where the nature of flow of water(or fluid) is guided by the pumping rate(or entrance velocity in the pumping well) along with the medium of fluid travel path - either because of inhomogenous - anisotropic nature of fractures or bacause of the fracture filling materals. So in all such cases the well loss may actually be an extended part of the formation loss itself attrubuted to the nature of the flow regime. And as such it may be comparable in analogy to that of well loss through casings in granular zones.

Next part of the problem in continuation to the previous is that, if I have to design a recharge bore in these areas - will be well loss factor(i.e.) remain same / increase / decrease if I design recharge rate to be some where around the safe / sustainable yield.

I would expect similar losses to the ones you measured in the pumping stage with similar discharges in the recharge stage..

In order to Discover the effect of singular fractures a recovery test is better than a drawdown test since the influence of the pressure reduction is visible when the water Level hits the fracture.

It is observed that the well is located in the consolidated crystalline formation , the well loss and fornmation loss is same .Because the bore wells are constructed upto a depth of casing only below the well is open what ever the water drained to the well is directly from the formation only.