In the LHC, we are talking about mini black holes of mass around $10^{-24}kg$, so when you talk about $10^{15}-10^{20}kg$ you talk about something in the range from the mass of Deimos (the smallest moon of Mars) up to $1/100$ the mass of the Moon. So we are talking about something really big.
The Schwarzschild radius of such a black hole (using the $10^{20}$ value) would be
$$R_s=\frac{2GM}{c^2}=1.46\times 10^{-7}m=0.146\mu m$$
We can consider that radius to be a measure of the cross section that we can use to calculate the rate that the BH accretes mass. So, the accretion would be a type of Bondi accretion (spherical accretion) that would give an accretion rate
$$\dot{M}=\sigma\rho u=(4\pi R_s^2)\rho_{earth} u,$$
where $u$ is a typical velocity, which in our case would be the speed of sound and $\rho_{earth}$ is the average density of the earth interior.
The speed of sound in the interior of the earth can be evaluated to be on average something like
$$c_s^2=\frac{GM_e}{3R_e}.$$
So, the accretion rate is
$$\dot{M}=\frac{4\pi}{\sqrt{3}}\frac{G^2M_{BH}^2}{c^4}\sqrt{\frac{GM_e}{R_e}}.$$
That is an order of magnitude estimation that gives something like $\dot{M}=1.7\times10^{-6}kg/s$. If we take that at face value, it would take something like $10^{23}$ years for the BH to accrete $10^{24}kg$. If we factor in the change in radius of the BH, that time is probably much smaller, but even then it would be something much larger than the age of the universe.
But that is not the whole picture. One should take also in to account the possibility of having a smaller accretion rate due to the Eddington limit. As the matter accretes to the BH it gets hotter since the gravitational potential energy is transformed to thermal energy (virial theorem). The matter then radiates with some characteristic luminosity. The radiation excerpts some back-force on the matter that is accreting lowering the accretion rate. In this case I don't thing that this particular effect plays any part in the evolution of the BH.
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I think, Kyle's link is a really good answer.
For they layman (like me), you can run some numbers here:
http://xaonon.dyndns.org/hawking/
a 1 earth mass black-hole (about the size of a ping-pong ball) would radiate so little energy that it would easily destroy the earth, even from a low orbit. It might take some time to devour it, but no question, it would destroy and eat the earth.
a black hole about 1/2 the mass of the moon (this would have the size of a small grain of sand, about 1/10th of a MM in diameter), this would be about temperature stable in the backround radiation of the universe, it would add as much energy as it would radiate off, but on earth, it would see much higher temperatures and would add mass, and in time, devour the earth.
But we'd see the effects long before then. A black hole, 1/2 the mass of the moon would exert 3 G forces at 300 KM, so that that means is it would effectively tear apart an Arizona sized hole, 300 KM deep, wherever it was - now it wouldn't eat that matter right away, but it would pull it apart. Deeper in the earth the effect would be smaller, but the hole would move around with relative ease, basically tearing apart the earth as it moved.
If we look at something more manageable, say, G-force would be 1 G and 1 meter, so it would only be very locally disruptive, like if that fell on your house it would tear a hole through it, but could leave most of the house standing. A 1.5E11 KG black hole, about the mass of 50 empire state buildings. A black hole like this would radiate significant heat, temperature nearly a trillion Kelvin (link above), but passing through the earth it might absorb about as much as it gave off - that's kind of a ballpark guess. Something around that size, in the range of a 150 billion KG. A black hole that size would have a life of about 8 billion years in empty space, and it might be able to eat the earth if it was on the surface/in the core.
I think, somewhere roughly in that range. It's worth pointing out that a black hole that size, it it was in space, it would likely just fly through the earth, not got caught in the earth's gravity and the damage would be far less.
Also, they don't think primordial black holes exist. They've given up the search.
http://www.nature.com/news/search-for-primordial-black-holes-called-off-1.14551
Finally, where it appeared wouldn't matter as much as what it's orbit was. Inside the earth it would interact with more matter than in the atmosphere. Theoretically a black hole in the atmosphere but at orbital speed, could stay in a stable atmospheric orbit for some time. In orbit, far enough away from the earth so it wouldn't absorb earth matter, there would still be potential tidal issues. A moon mass black hole in orbit, 10 times closer than the moon (24,000 miles) would have 100 times the tidal effects than the moon currently has - the oceans would rise and fall about a couple hundred feet with each orbit. At 24,000 miles it would be close to a geosynchronous speed, so you'd only see the high tide every 5 days or so, but that much tidal force might make the earth close to unlivable. Earthquakes and Weather changes.
Hope my layman's answer isn't too wordy. Interesting question.